I can add a data point to being able to select seats. I have a DCA - BOS shuttle scheduled for Friday morning 630am and was anticipating just getting up a little earlier Thursday to check-in and snag a seat (flew DCA - LGA shuttle for 5 weeks this summer so I'm used to that routine). Was actually checking just now because I still need to get my UA and KTN into the booking and while I was unable to do either of those things I decided to click on the "seat selection" for fun and it actually popped up and showed me a 1/2 full plane. Rows 6, 7 (except middle), and 12 (aisle only) are all available for $43. The rest of the rows (4, 5, 8, 9, 10) are all grayed out as are all of the aisle seats all the way back to 18. I got lucky and I guess snagged the last aisle seat in 19. Needless to say they clearly opened up the seat-map, I'll update if for some reason at T-24 all the grayed out areas show available again.
Note I am not a US elite (not logged into my non-elite US account either) and I still don't have my UA 1K status attached to the ticket so it's clearly not limited to US elites or *G since currently my ticket reflects neither of those things.