I love the idea. Squad Leader or Tank Commander could call it in just like they do with artillery or the new mortar & rocket strikes. Would be even better if tank ammunition crates can be dropped as well.
But then, respawn is exactly the same thing because you get new troops with full ammo.
Resupply missions weren't something you could really call in during the encirclement. Seems to small a scale to happen real-time. You would expect the ammo to be airdropped behind your own lines, and not in the no man's zone. But who knows, I'm sure boxes could get dropped in the wrong area.
Could just have it done with static resupply boxes that look like they were dropped previously. It would be cool to see them falling in, though.