Not sure if such program exists or you have the desire to accumulate miles aside from UA, you can search and see if other *A FFPs may allow retroactive credit for flights more than 6 months ago.
Thanks, but since I will be based in DEN in a month or so, UA M+ is pretty much it for me. (Thanks to the help on
this thread.) The only other program I'd consider is SQ's Krisflyer, but I'm far more likely to be on UA than any other airline at this point.