RetroPie builds minivmac/minivmac, winterheart/quasi88 and TurtleBazooka/px68k under the package names minivmac, quasi88 and px68k (scriptmodules/emulators/minivmac.sh:12, quasi88.sh:12, px68k.sh:12). With built_by retropie and those names in cores, RetroPie resolves them to minivmac37, quasi88kai and px68k-sdl, while its libretro builds stay reached through retroarch. Measured: RetroPie keeps 532 of 532 platform files OK and gains the 14 files the standalone builds read; no other platform's resolution changes.
The ROM list is src/memory.cpp:87-100. A missing ROM is filled with 0xFF and the emulator starts, so only N88.ROM, N88SUB.ROM and N88EXT0.ROM, without which nothing boots, are required. The directory is QUASI88_ROM_DIR, else <home>/rom (src/file-op.cpp:444-521). The CMake ROMDIR setting only appears in the help text.
Abdessamad Derraz