Currently you can already play MSX-AUDIO music on OPL4, but without samples. It would be nice to decode the ADPCM to PCM and play those back on the OPL4 as well.
Because the MSX-AUDIO can freely play samples from any start and end position in its memory, while the OPL4 requires start, end and loop positions to be specified in advance in the wave headers table (which can’t be accessed during playback), this would require a pre-scan of the MSX-AUDIO VGM data to determine and populate the OPL4 instrument headers prior to playback.