I just did this recently to meet an elderly relative connecting through EWR to an international trip. I booked an AA ticket that morning for a late flight that evening. No problems from TSA going into the "wrong" concourse at Terminal A to meet the incoming flight (and I got Pre-Check). At both the Terminal A lounge (while I waited for the incoming flight) and at the C3 Terminal C lounge, I tried entering with just my UC card. Both lounge agents insisted I needed to show them a BP, but they didn't react to the AA BP I handed them. They scanned them and they got the same single beep as a UA BP, so I assume UA is capturing/validating that it's a legitimate pass somehow.

Just before I left the airport (but well before my reserved flight's departure), I called AA and canceled the ticket. Only hitch was that once they canceled the flight, I had to go to the AA site and request a refund - the charge and the credit both appeared on my credit card statement within a few days.