Fossils ... ugly head ... yes. And it is something that has to be accounted for eventually but that may not happen 'til after discovery is official. We had proof of chimps long before we had fossils of chimps.
I'm not so inclined to look for high altitude burial, deliberate or otherwise. There isn't a lot of food up there so a BF in those locations was probably traveling through, not living there. An older, unhealthy one probably couldn't get up there to die and be buried. Unless they're packing their dead a long ways for burial, I think low elevation burials are more likely. Lahars which travel a long distance from the mountain source remain good candidate areas to look because anything caught in the stream bottom, even miles from the mountain, could be killed and buried. The lower Toutle and Cowlitz rivers from Mt St Helens, the Sandy River from Mt Hood, and maybe the upper Rogue from Mt Mazama would be places to look. Some places with very heavy ash-fall could hold fossilized remains, however, accessibility is an issue if they're under 30-50 feet, or more, of ash.
It's a needle in a haystack search.
MIB