I'm not an expert on balance, but Empty Palm seems a bit on the strong side. Just on its own merits, it's nearly auto-pick, IMO, without getting into the fact that it's an MBA, which effectively saves you a feat if you want to have a decent OA. At will immobilize that can be used as an MBA feels cheese-able.
You would be the Warlord's best friend and/or dream date, especially if you have stances like Masterful Spiral, so they don't even need to be adjacent to you.
It might be strong enough to consider adding a requirement like "must have both hands free."
Maybe, but consider the other At Will powers usable as MBAs (unrestricted) for other classes and the extras they get:
Paladin, Virtuous Strike, MBA, gain a +2 bonus to saving throws until the start of your next turn.
Fighter, Wicked Strike, MBA, Extra Con damage (double at level 21)
Cleric, Sonnlinor's Hammer, The next attack the target makes before the end of your next turn takes a penalty to the damage roll equal to your Charisma modifier.
Sorceror, Ensorcelled Blade, If the target hits or misses you with a melee attack before the start of your next turn, it takes damage equal to the extra damage from your Spell Source class feature.
Vampire, Vampire Slam, you can push the target 1 square.
Druid, Grasping Claws, the target is slowed until the end of your next turn
Druid, Savage Rend, you slide the target 1 square
There are a fair number usable only when charging or making an OA,
notably:
Fighter, Grappling Strike, MBA for OA only, you grab the target. The grab ends automatically at the end of your next turn.
It seems to be balanced against those, I think, especially when considering that the movement part of the full discipline is not really very impressive compared to others. It's ... interesting, certainly... but rather limiting in that you must end up next to the target.
The damage needs to be d6 though, not d8, due to imposing the immobilized condition and the fact that this can be used as an MBA.