Honestly – I have attended a rivalry game at Rice Eccles wearing BYU gear. I was only mildly heckled as I walked all the way up the student section steps. It was fine.
I think most of the ease of our experience had to do with the fact that we were on the very back row with no one above us.
I was the smallest guy in my group at about 6'3" 230, and we were in our 20s. That might have helped dissuade a fan or two from getting in our face, but perhaps not. Other than being pretty big dudes, we'd have looked rather harmless. Whatever the case, we cheered enthusiastically throughout the game, which BYU won, and really had no issues.
I doubt that I would have the same experience were I to go into the middle of the Ute stands now, but my only actual experience was fine.
That said, I don't see a whole lot of Utah fans taking on that challenge.