or is it just kind of a what has she got to lose thing by hoping he'll go for the gold?

IF he does for the gold, I think it would be a huge misstep on the show's part. I know we've established Bron puts himself first, and "fights without honor" and all that, but I don't see him as an entirely unprincipled trash human who would sell out Tyrion and Jaimie like that.