There are some emulators based on Neko Project 2 that are open source and could be ported to libretro.
The official emulator site is here.
Xnp2 Originally a port of Neko Project II to Linux, the author has been improving 386 processor emulation. Source + Windows binary available. ( the source file is xnp2-0.84.tar.bz2 )
Info pages:
Nice little FAQ about Neko Project 2 Wikipedia page about PC-9801 Website full of Games preview with pictures