Good morning!
I did the P3-ROC integration for the Capcom Kingpin remake. I started out trying to do what you aim to do but eventually went the emulation route because I only had several weeks to get the game ready for TPF.
Capcom systems at the time were quite a bit more advanced than their WPC counterparts. Capcom used a 16 bit data bus through their board system compared to WPC which used 8. The theory of operation between both systems is pretty much the same, however.
I imagine the choice not to support Capcom natively at the time the P-ROC was launched was mostly in the interests of balancing FPGA logic complexity with demand (tons more Stern games out there between SAM/Whitestar compared to Capcoms).
Happy to help however I can though. I've got a test fixture built right behind me for the Capcom system.
