Ah, looks like they recompose the songs in a tracker. Basically you use something like Hoot to sample each instrument into individual note samples, load them into your tracker of choice (like
OpenMPT), and used them to recompose each channel.
Now, if there _were_ a PC9801 sequencer available, I'd be very interested in it. =O
This is something I'm working on right now, it's Love-coloured Magic. It's still very much incomplete (have to redo the RSS and first two SSG channels), but it sounds pretty good so far. It's an IT file, You can load it in MPT as well, 'though I don't know about older trackers (even Impulse Tracker may not load it =/). I sampled this via Hoot.
( Only for release within MoTK, btw. =X I cut the sample quality down enough so I could upload it here, so it won't sound okay until I finish it and release the full version. )