I probably won't be writing any more expansion sound chip emulators. As I remember, MMC5 and VRC7 are rarely used. I was going to support FDS a while back but couldn't find reliable documentation. Having never even heard the thing, I didn't go further.
I care most about exact emulation; if I can't get very close I won't bother. N106 and VRC6 were simple enough that I implemented them based on documentation alone. GBS sound is still somewhat problematic because I don't have a Game Boy development cartridge for testing the sound hardware. Documentation always falls short in some way and it's not fun going in circles trying to figure out what's supposed to happen in ill-defined cases.
It might be possible to run other FDS, MMC5, and VRC7 emulators in parallel. The simplest solution of course is to include an alternate NSF engine that supports everything, and use it when the expansion sound bits in the NSF header have any bits other than 0x11 set.
As an alternate NSF sound engine with support for (I think) all the expansion sound chips, I'd recommend
Disch\'s NotSoFatso , or the band-limited rewrite in his experimental Schpune. I think they're both under the GNU GPL license (I'm e-mailing the author about alternate licensing). I'd even volunteer to separate the NSF playing code from NotSoFatso or Schpune into a more independent player library.