Here's the fact I know:
- UTR#50 draft #5 sets Tr, and the proposal from MS was Tr too.
- All the today's font don't have vert for this code point, so it means it will result to U at least for a while.
- Having Tr can recommend future font developers to put sideways vert table. This can lead to inconsistencies by when the font was designed.
- MS Word sets it U.
- fantasai believes it's U.
- One vendor I know believes it's R.
Given #2 and #3, I don't think having Tr is a good idea. If I understand correctly, the spec says Tr is supposed to be where the most current font have vert, and it contradicts with it.
I have mild preference to U given #4 and #5, but R also makes sense if we consider consistency with U+FF1D FULLWIDTH EQUALS SIGN.