Don't know about the D5100, but focusing screens are generally easily replaced parts. Some cameras have optical focusing screen accessories to replace the original focusing screen (e.g. if you prefer to see a "grid" pattern when you look through the camera, they make focusing screens with grids on them.)
I wouldn't worry too much about damaging it during cleaning. It's not in the "image path" of the camera (you could go ahead and put a serious scratch on it if you wanted and it wouldn't affect your photos -- it'd be annoying to have to look at when you use the camera, but your photos would still be fine.)
Given that it doesn't affect your photos and generally is a very easily replaced part, this is something you could probably do yourself (it's low risk.)