@thrussmeister
I haven't given linux configuration software my full attention. I used WinIPAC to configure my older IPAC4, but you might not need to configure anything--unless you are planning to re-flash the firmware. The IPACs default to sending MAME keys for each player (although newer firmware apparently looks like a joypad instead of a keyboard).
I have done a LOT of per-game arcade configs on my 4-player cocktail setup and it's definitely easier using the actual pushbuttons and sticks as wired instead of trying to remember what control sends what key. So, I would say do this:
Figure out KEYBOARD vs. Joypad firmware first. Get support from Ultimarc as needed.
a. Deal with any Ultimarc configurations using WinIPAC if you need to.
Wire the IPAC
Configure emulators on the Pi.