Dremel did the trick
I did some light sanding, grinding and brushing with a Dremel tool, that worked perfectly. I needed about .9mm in thickness and .1mm in width. After about 30 mins work, I was able to fit my P5100 into a dock belonging to a P7500. It now slips in and out perfectly, and will not damage the finish on the tablet.
If you're not confident with a Dremel, you could also use some low gauge sandpaper, which will also work, since there's very little to be adjusted. Be sure to concentrate on the parts protruding from the surface for the corners of the tablet.