I've done a buy-up while standing in the blob to board... I forget why (perhaps just to see if I could), and it was domestic out of IAD, but I very clearly remember looking up at where I was in the UG list relative to how many open seats there were, looking at the offer in the app, and deciding "why not, that's worth it to me"... I think I may have even shown as a cleared UG even though it was a buyup (but display issues are a different thread)