Total Rewrite: My previous answer was lackluster at best. Let's try again.
Third time's the charm: As you pointed out, there is a way to get the thunder keyword within RAW. The Malec-Keth Janissary paragon path that gives you 1d4 thunder damage to all of your attacks. You'll obviously need to MC swordmage to do this.
Now you can start getting creative.
- Mark of Storm: Adds a slide 1 to all your thunder attacks.
- Arcane Implement Proficiency: Choose orb.
- Orb Expertise: Adds 1 to all your slides with orbs.
- Petrified Orb...: Adds 1 to all your slides with this orb.
- ...of Unintended Solitude: Nice little daily power to zap an enemy out of combat for a turn.
- Hamanu's Terrible Roar: Adds deafen (save ends) on thunder attacks.
Now you've got a guaranteed slide 3 + deafen on all targets you hit with Hand of Radiance. If you need a little extra, toss in a Stormglass Shard for another 6 squares of sliding.
If you're looking for other ways to enhance your controller-ness, take Astral Wind as your other level 1 at-will. Then you can pick up Rushing Cleats, Gauntlets of the Ram, Ring of Ramming and Resounding Thunder to give you a Close Blast 4 that pushes 6 squares and slides 3 squares.