Picked this up a month or two ago but haven't had a chance to ride it much. I haven't really been able to find any info on this model, I'm guessing it was a European model. The serial number starts with a U which should make the frame a 1992, but by then all of the USA mountain bikes had moved to the Elevation naming. It has all Deore LX components (aside from the DX shifters) and they're from different months in 1991.
Edit: Also, it looks like the 100 logo on the chainstay has what is apparently Dutch writing:
Miyata, 100 drs inbyggt cykellkunnande!
It seems very similar to the Miyata "Century" MTB from the 1990 catalog (it even has the 100 decal on the chainstay) but without the gold lugs and the fork has mid-fork low-rider rack mounts. Kind of a mix between the Century and the Trail Runner of that year, but with different decals. If anyone has any non-US miyata catalog scans or has seen one of these before, I'd love to have some more information.
I took it out for a ride today on some dirt trails in Balboa Park. I was considering selling it but it felt great and was a lot of fun so I may hang on to it for a while. Here are some pictures when I first got it out this morning, kind of dusty from sitting for a month or two.