There used to be a side-release of OPL that had a "PS1-GAMES" tab in it, but the way it works is that OPL would basically just read a list of *.ELF shortcut-links to VCD files in separate partitions.
The "compatibility of OPL with PS1 games" as you say it is fully dependent on the emulator that...
The FreeHDBoot form of FMCB was always the "sexiest" form of the mod, but I must say that with exFAT and its ease of use, plus the fact the FMCB boots even faster, I think FHDB is now abandon-worthy.
The only problem is that some Games don't support VMC well, so when you have FMCB in slot 1 and...