in that particular case, the top part of the case is the plate. it's a sandwich style case. pcb would just be suspended under the switches by the switches. might make it difficult to use the hot swap....
also, have no idea if those two items will work together. isn't the bottom row of a HHKB a little different than just taking off the outside corner modifiers (CTL)? larger spacebar? or is that winkeyless? (larger space bar)