Make sure they know the rules
One reason to overconserve powers is simply being unaware of the rules. Remind the players when the powers recharge, and that there's no cost in using the powers if they can recharge them right after the battle.
Necessitate it
In Finland we have this saying that translates roughly to "Siberia will teach 'em". It sounds like you already tried by throwing a suitably hard challenge - don't give up, they'll learn eventually. You shouldn't pull your punches simply because the players aren't exploring the full spectrum of their tactical options. If you can get them to use their dailies and encounter powers as an emergency resort first, they might realize their usefulness to prevent emergencies as well.
In particular, you could aim to create situations where the players' unlimited attacks are significantly lacking. For example, if the only area attacks of the party are encounter and daily powers, let them face an encounter against numerous weak minions. Conversely, you could create a situation where an effect of one of the daily powers is tailor-made to accomplish a task: if you want to get your cleric to experiment that power that strikes enemies mute, have them fight a monster that primarily attacks through sound.
React positively to the use of powers
My players react very positively to embellished descriptions of the effects of their power, and this is a good way to reward their use if mechanical rewards are not well-suited to your homebrew system. So when you actually get someone to use their limited powers instead of the unlimited ones, make that moment cool. Embellish, put them in the spotlight for a moment. Describe the severed goblin parts, or the black knights buckling before the heavenly light. This reinforces the idea that the powers are cool and meant to be used.
Lead by example
Tag an NPC with your party, or have one fight against your PCs with abilities similar to theirs. Let them use their dailies and encounters to show off their powers, and serve as examples to your PCs. Nothing loosens a mage's fireball trigger or a paladin's righteous fury like another character trying the same, especially if the other character is a hostile.