The way I look at it, is it really shouldn't have to be up to the tank to ask the healer if the healer needs tank gear. It should be up to the healer to speak up and say something. You may get some nice person who will pass on said gear, and you may not.

If, like you're saying, the healer's alt tank isn't high enough to tank the dungeon, why not go in with friends or FC members rather than relying on random people in the DF to cater to you?

But, overall, I agree communication is the key.