I wonder how easy or time-consuming it is for a SPG agent to check whether one is registered for a promo? I notice that when I call, the agent frequently prefers to check something using the confirmation number for the hotel reservation, but perhaps instead finding things through the account number would make the promo registrations more obvious.

OTOH we can't easily check for promo registrations ourselves and at least some promos (such as the current extra points offering for North American LMs) don't send a confirmation email either. Wouldn't it be nice if we could click on a link on our main account page and see our list of current promo registrations, ideally with links to their T&Cs, FAQs, and lists of participating/nonparticipating hotels? Having this visible in the account might have served as a warning to the OP to check whether registration would be required.