The companion is also supposed to be with you when you talk to the human being about the upgrade. I was meeting my sister at EWR recently, and tried to comp-up her while I was waiting for her NJT train to arrive, but the agent wouldn't do it without my sister there showing ID (even though I already had her BP from doing OLCI and we were on the same PNR).

There might be exceptions to that rule, though. I'm not sure.