Presume that the Heathkit PAM is like the Morrow Keyed-Up 8080? (e.g., see:
https://archive.org/details/bitsavers_morrowsche77_699793). Depends on ROM+CPU for keypad support. The idea here is CPU-independence, which would be the way to go. But as John notes (later) probably will take a new board to accommodate the necessary circuitry -- the functionality isn't all *that* complicated, but it's still non-trivial to substitute for that CPU+ROM and "simple program".
Perhaps we could treat the keypad-function as a self-contained pendant (thus gaining space off-S100-board), but then one needs to deal with a lot of data/address long-lines that need to be buffered close to the backplane. Could replace the long-lines with a serial protocol with shift registers at both ends? Getting complicated ...