Hey all ... from the "in case anyone is curious" department ...
Starting with a black, blank-key HHKB Pro 2, I bought a set of blank-key white/grey keycaps, and put together a custom layout. I pulled in the grey modifier/return/delete/etc. keys, and the white number keys, and kept the rest black. Here's what it looks like:
Personally, I'm thrilled with the result, although I do wish that the grey was darker. (Well, if I fully had my druthers, the white/light-grey would be dark-grey/medium-grey, and a red escape key would be cool as well. But I'm happy with what I have.)