I think because of the fare basis and lack of codesharing on UA/BR through ORD you're better off going your original route. Buy separate tickets and get the UA agent to interline the bags. It's definitely possible and it sounds like you've done it before.
Depending on the day you're flying the last RDU-ORD flight on UA arrives at 630pm. The BR flight doesn't depart until 12:20am the next day so you wind up with an almost 6 hour layover. That's plenty of time to even collect your bag in ORD and then check it in again at BR. Of course, that's potentially a bigger hassle than just getting the UA agent to interline the bag.
Or, just book the through ticket on the SFO route
BR flies the same 777-300W through SFO as they do for IAH and ORD. I would take that flight over UA any day, especially if I could book it on a 016 ticket to get the RDMs. BR has a fantastic J product and service if you're flying business.
-RM