I know of Tyrannical Threat that can curse everyone in a Close Burst, but are there any other powers that let you curse everyone in an area like that?
In addition to AoE Curse options, I'm including basically anything that lets you apply your curse more often than normal, since that can still speed things up.
Eyes of the Vestige - Level 1 At-Will
Target: One creature
Hit: 1d6 + Constitution modifier psychic damage. Choose the target or a creature within 3 squares of the target and within the target's line of sight. You place your Warlock's Curse on that creature; [...]
Not a mass Curse power, but it can double the normal speed at which you apply curses by letting you place a curse on a nearby enemy when you hit with it. Including the one per round you can do normally, this bumps you up to two per round that you use it, and with a bit more freedom on who to curse.
Even if you're not a Vestige Warlock, a human can still pick it up as their third at-will power. Alternately, you can pick up a Master's Wand of Eyes of the Vestige, which lets you use the power once per encounter, and also bumps the cursing range up to "creature within 5 squares."
Tyrannical Threat - Level 5 Daily
Close burst 3
Target: Each enemy in burst
Hit: 1d10 + Constitution modifier psychic damage, and you place your Warlock's Curse on the target.
Miss: Half damage.
Infernal Pact: You place your Warlock's Curse on the target.
You mentioned this one yourself, but for completeness's sake, including it here. Lets you apply your curse in a close burst 3, which can hit a lot if you can get into the middle of a lot of targets. If you're Infernal Pact, it even applies the curse hit or miss.
Fate's Frayed Thread - Level 6 Encounter Utility
Trigger: An enemy misses you with an attack
Effect: You place your Warlock's Curse on the target. [...]
Very straightforward, and an easy enough condition to trigger. Hard to complain about a free curse every encounter.
Infectious Curse - Level 9 Daily
Target: One enemy
Hit: 2d12 + Constitution modifier damage. Until the end of the encounter, any enemy that starts its turn adjacent to the target gains your Warlock's Curse.
Infernal Pact: You can also slide the target a number of squares equal to your Intelligence modifier.
Make one target a beacon of cursing for the entire encounter. Infernal Warlocks also get to shove the target into the middle of a group of enemies to maximize the effect they get out of it. It's even pseudo-reliable, allowing you to take a bit of damage if you miss to prevent expending the power and getting an attack bonus if you try again.
King's Step - Level 10 Encounter Utility
Trigger: An enemy misses you with an attack.
Effect: You fly your speed and are insubstantial during the move. After the move, you place your Warlock's Curse on the triggering enemy.
Pretty similar to Fate's Frayed Thread, though with an escape attached to it too.
Break the Will - Level 13 Encounter
Target: One creature
Sorcerer-King Pact: If you spend your fell might [...] you place your Warlock's Curse on each enemy adjacent to the target.
Pretty much requires Sorcerer-King, whether through your starting pact or Twofold Pact, but it's an Encounter power instead of a Daily.
Evil Expulsion - Level 15 Daily
Target: One creature
Hit: 4d10 + Constitution modifier psychic damage, and the target falls prone. You can place your Warlock's Curse on any enemy adjacent to the target.
Infernal Pact: You can instead place your Warlock's Curse on any enemy within 3 squares of the target.
For our purposes, basically a weaker form of Eyes of the Vestige. Obviously is available to everyone, has better non-curse-related effects, and is pseudo-reliable, but you probably have better options.
Balican High Praetor [Greater Pact]
Prerequisites: Warlock, Sorcerer-King Pact class feature, Balican Praetor feat
Whenever an enemy you have cursed drops to 0 hit points, you gain the following benefit in addition to regaining the use of your fell might. The next ally you can see that hits with a damaging attack before the start of your next turn subjects its target to your Warlock's Curse. [...]
Along with a number of other effects, this can give you some massive off-turn cursing ability if you happen to have an ally with big AoE attacks like a Wizard. It does require another feat, and the Sorcerer-King pact, but it can be powerful in the right party.
Prerequisites: Tiefling, warlock, Warlock's Curse class feature
When an enemy hits you, you can use your Warlock's Curse as an immediate reaction against that enemy, even if it is not the enemy nearest to you.
Counterpart to the utility powers listed above, with the bonus of being at-will instead of once per encounter. Obviously, getting hit isn't something you want, but this can help make up for it when it inevitably happens.
When an enemy cursed by you is reduced to 0 hit points, you can use your Warlock's Curse as a free action.
Kind of a lesser version of the Rod of Corruption, you can fling out another curse when you drop a cursed enemy. On the plus side, you don't have to give up your Pact Boon or one of your hands.
When you use your Warlock's Curse class feature, you can curse the two nearest enemies.
Another nice doubling to your cursing speed, available to anybody and easy to use. You'll probably want to grab this one anyway.
When an enemy cursed by you drops to 0 hit points, you can curse all enemies adjacent to the target as a free action.
Pretty straightforward spreading of curses when an enemy dies. You can probably do better by now, with everything else on this list, but it's there if you want or need it.
Requires Star Pact. It's not much in the way of curse spreading, but it has a Daily utility that lets you apply a curse to one person nearby, along with its other effect. If you've already got your curses spread around, though, the action point feature is pretty awesome.
This is probably what you're looking for here. Centered entirely around having lots of enemies cursed. All three powers let you fling around extra curses, and the encounter attack is even a friendly close blast 5 that curses everybody (before the attack, even).
Requires Intimidate training. The level 11 feature lets you fling out another curse at the nearest enemy if somebody else kills one of your cursed targets, or give that enemy a debuff if they're already cursed. Nothing special, especially compared to Hexer.
Requires Vestige Pact. The encounter utility lets you prevent certain effects and curse the guy who tried to hit you with them. Situational, but nice when it comes up.
Rod of Corruption [3+]
Lets you give up your Pact Boon for a big burst of free curses. Available early, and you don't need to attack with it, so you can just keep it in your off-hand. Solid at the beginning of a fight, less useful later on.
Armor of Dark Majesty [4+] (Leather or Hide)
Minor action Daily power to let you throw out (another?) curse on someone other than the nearest enemy. The +2 item bonus to all defenses against cursed enemies is probably the more attractive feature (especially for a mass-curser), but the power's helpful sometimes too.
The Rod of Corruption does what you want but only when your pact boon is triggered.
Rod of Corruption
Property: Whenever your pact boon is triggered, instead of taking its normal benefit you can transfer your Warlock’s Curse to each enemy within 5 squares of the original target.