Since the OPLL is a derivative of the OPL2, we should be able to emulate the OPLL on the OPL3 and OPL4.
Although the exact settings of the OPLL’s built-in instruments are not known yet, emulators contain pretty decent approximations which could be used. Once Wouter completes his research on the exact instrument ROM settings it can be updated to use the real values.
This issue could benefit from some user feedback, because currently for me this does not have a high priority because MSX-MUSIC cartridges are widespread among MSX users.
One reason to do this would be to implement support for the VRC7 (#30), which is identical except for using different built-in instrument settings. However the only VGM pack with it is Lagrange Point and that one also uses the NES APU, which is an entirely different beast to emulate (don’t see that happening).
Having listened to (and being very impressed by) the Firehawk HDD soundtrack being played by the OPL4, I would very much like to have at least the possibility to listen to OPLL music with OPL4. Synthesis quality is better and you also have less noise, which is neat.