I can see from the Github source that the ROMs are built into the core by means of creating an MIF file using srec.
So, 2 questions...
1. If I wanted to use different ROMs I presume I'd have to rebuild the entire core? Unfortunately I don't have a build environment set up at the moment.
2. Are there any "gotcha's"? And by this I mean will other ROMs even run? For example, I see "Bank 8 is special, in that B600-BFFF is mapped to RAM (for SWMMFS)" so does that mean that SWMMFS has to be in bank 8? Nothing else could live there instead? Not really a problem, there are plenty of other banks available.
I'm just trying to gauge what is and isn't possible if I wanted to use the core beyond a simple MMC based games jukebox.