For MAME I use NECCDM.SYS (24369 bytes) and it works pretty well. IIRC I tried to run Policenauts once and the installation worked, but the game crashed on boot with some kind of error. I'm not even really sure if that was due to the emulation or if I just screwed up while setting up the appropriate drivers, since it's a ridiculously finicky game even on real hardware.
I don't have anything to contribute, other than, thank you for sharing that info. Later systems like the PC-98 or even regular PCs had no end of compatibility issues, and having confirmation of a game being unstable is intensely helpful for anyone working on emulating such systems, so they don't spend a lot of time trying to solve weird bugs that actually happened on real hardware. MAME could definitely benefit from more people like you.
No prob, my PC-9821Nw150 is always available to anyone who wants to test anything on real HW. I don't have any older models to test, though. I only got that one because, being a laptop, I could get it sent to me from Japan more or less cheaply. That, and it's one of the last models with an actual FM synth.
By the way, I tried again and now I remember what happened: Policenauts requires the AVSDRV.SYS PCM driver, but it has to be a version that works with your specific hardware (I remember having to search everywhere for one that worked with the Nw150), and I couldn't find any that worked with MAME. So in the end I always get something like this:
Which would be something like "the PCM driver AVGSDRV.SYS isn't installed".