Hmm, couldn't say why it works for me and not for other people now.
The rooms I booked are at Harrah's Las Vegas and the Rio. Basically the cheapest ones I could find, though I had to alternate between places since I apparently bumped into some maximum reservation limits as I already had comp rooms booked at Harrah's and Rio in November. I figure if I'm going to drive my player rating into the ground I should book as many freebies as possible while I still can.
I double-checked my confirmation printouts, the promo code was applied. It wouldn't give me any comp rooms with this promo code, whereas I could get comp nights without it, but right now this is my cheapest, lowest variance, lowest effort way of getting those 7000 tier credits before the end of the year.