I'm glad I got back to using MZ-700 avatars.
.fdd and .hdi files? You sure you aren't trying to run PC-98 abandonware on a PC-88 emulator? All the PC-88 files/dumps I've seen are .d88 or something unlike what you've specified. I wonder if that music disk you mentioned is from the Firecracker doujin series...great stuff.
For stuff in English on any J-PCs, you're more in luck these days since a fan translation scene's finally gotten on track. Games like the original version of Enix's E.V.O. and Brandish 2 have either public or beta patches ready to go if you have the game ready to play on emulator (there's ways to patch the original floppies as well). Action-based games usually don't require Japanese knowledge to progress, which is why Night Slave's often mentioned as a PC-98 recommendation (you can turn off the eroge content in the options menu).
J-PC software developers, since at least 1984's Eiyuu Densetsu Saga on PC-88, used some kind of artwork scanning/digitization method followed by algorithms and pixel-by-pixel tuning to accurately replicate and dither their in-game materials. Some game graphics were best suited for pure pixel work, but these are way less in number vs. the horde of erotic games which relied on scanning first and tweaking later.
Yes, i think i got 5 PC88 games early this morning from abandonia and they were all .hdi or .fdd. I was just trying to run anything before starting to look for english or translated games...
Thanks for the other tips. I will check on Night Slave. Can you point me to a better site than abandonia to get the games in .d88? (once again, since abandonware i think we are ok with #antipirategaf)