I think the reason TH1~5 drums can't be dumped is because they're not FM commands. They're just special commands for the OPN/A chip to play the hard-coded drum sample. If I remember correctly, generic MML doesn't have the concept of percussion tracks.
If someone wanted to, they could probably write a method to interpret the percussion commands and turn those into MML strings. Also, it's worth mentioning that TH1~5 percussion is actually easier (in my opinion) to interpret since they're not FM drums, which are usually chip-specific.
On a somewhat different note, this is why PMD-to-MIDI conversion works very well. The only quirk is that all the instruments get converted to pianos, although that can be easily changed in any decent MIDI sequencer.