Koji informed me of the call today, which I won't be able to make (time difference in Paris).
My colleagues who are implementing vertical text in a WebKit application inform me that they would prefer these characters to be rotated 'R':
0xFF0D // Fullwidth hyphen-minus
0xFF1A - 0xFF1E // More fullwidth characters: colon, semicolon, less-than sign, equals sign, greater-than sign
0xFF61 - 0xFFDC, 0xFFE8 - 0xFFEE // Halfwidth CJK characters
I understand that at least some of these are being considered for 'U'. We have feedback from JP publishers that 'R' is more aesthetically acceptable and that 'U' might introduce other publishing peculiarities. I'm writing this comment, thus, to support Koji and fantasai in proposing rotating half-width forms to 'R'.