If the tape is masked off by the matt, it shouldn't show up. But it might leave marks on the paper itself... Hmmm... gotta go think about this.
But then we still have the same issue of the matt is raised and the glass won't be in "contact" with the neg. I'll have to try the tape idea this weekend on some 120 film. My guess, is that i'll be okay with RC paper but for fiber, perhaps taping a used piece of fiber paper and taking it off a few times to remove some glue, may make it just tacky enough.