Hi B&V,
Welcome!
Your typical rustoleum/krylon is good to about 165 degrees or so, which should be fine for the plate, though I have no idea of how the paint will age in terms of color changes. The thing that usually get the hottest is the power transformer, and I prefer to use the rustoleum extreme heat ultra gas grille paint -- but it only comes in flat and gloss black as far as I know. It's good to 1000 degrees, which is somewhat overkill, but I certainly don't have to worry about it. Maybe use the gloss and scuff it with scothbrite or something like that. Or you could have things powdercoated, which is good to 400F or so, which is a very large margin for BH gear.
Aside from the power transformer, the hottest parts on the plate will be near the output tubes above where the big cathode resistors are located, but these plates are also fairly thick aluminum and heat tends to even out over a fairly large area. Also be sure to use footers at least as high as the stock BH ones and allow for plenty of airflow around and under the amp -- i.e., don't put it in a closed cabinet.
So far everybody who has built this amp really seems to love it -- I expect mine to be coming online soon after our pending move to a new home.
Good luck,
Jim
Jim Rebman -- recovering audiophile
Equitech balanced power; uRendu, USB processor -> Musette DAC -> 5670 tube buffer -> Finale Audio F138 FFX -> Cain and Cain Abbys near-field).
s.e.x. 2.1 under construction. Want list: Stereomour II
All ICs homemade (speaker and power next)