powder-coat is not brittle/flaky at all, it's very flexible if just going with the straight poly-ester type, which is what us backyard bullet makers generally use. I haven't seen powder-coat come off of anything yet. I have seen it burnt off and grooves that rub it off due to excessive friction when running super. but for a pretty slow heavy round ball, it'll stay right where you put it. You won't be shaving it off either, since it's just covering the ball, when you running it through a sizing die, it just slides through and allows the lead to shape under it. I am only making an assumption, but i'd think it worth a try since it's such a simple process. if it interests you, i could coat a few and send them to "someone" that has a BP and has a hankering to try.
Hi-Tek seems to be harder and more brittle, although i don't have more experience than one box of 500, it definitely wasn't soft enough to push your fingernails into it like PC, but since they actually get run wit supers it may be an even better route if it resizes the same.
then there is the thermoplastic powdercoating, where the bullet itself is heated to the melting point of the powder, generally nylon, which will "fusion-bond" with the metals, as it cools it will harden some i would suppose. I wasn't around guns when the NyClad's were the big thing, so i never got to feel it and see if it was soft or not. I am only making an assumption that NyClad's were coated using this method, they may have not.
if powdercoating balls, i guess it might be best to use a .001 undersize ball and fill the void with a heavy coat. that way you'll only be squeezing the poly which would make a great seal, i think. guess it'd have to be tried both ways.