We are leaving on SEA-GRU-EWR MR on UA metal tomorrow. For U.S. passport holders, OLCI requires entry of information on a BRA visa in order to complete check-in. My guess is that the same computer logic will cause a ticket counter agent to ask for a visa, too. Even though it's a through ticket, the computer is seeing it as a trip to GRU (in fact it checked us in through to GRU but not beyond.) So if you try to do it without a visa, I expect you will have to convince an agent that you can do that, and expect some argument or scrutiny, and perhaps variability based on which agent you get.

I suppose you could make up a visa # to fool the chicken, but they'll ask to see it at some point (and it only issued an International Card showing the check-in but not full boarding passes). Our visa # is 6 digits followed by "MD" and is good for 10 years. E.g. 123456MD.