So to report the result: Yes, not a problem, and no questions were raised when the still-not-cleared standby BP was scanned at two clubs at that hub.

OTOH, returning LHR-IAD-LAX on Econ, LAX denied me entry as I was purely arriving. I had no checked bag and was going to get re-caffeinated for the drive home. From reading the rules, I think the club agent was wrong, although the phrasing for *G "departure airport can be different from that of the United Club location being entered" is not as strong as that for J/F "may access United Club locations at departure, connecting and arrival airports along their eligible same-day routing".