Brian played one half of the Professor Quirrel/Voldemort combo in A Very Potter Musical. The part that is not a fragment of a soul, but the part that just wants to plant flowers when he takes over the world and misses the crazy serial killer when he is no longer attached to the back of his head and wonders if OK is good.
In the sequel, Brosenthal didn't get as big of a role. He was Seamus Finnigan and James Potter, both smallish parts.
During Starship, Brosenthal was still in school and so he wasn't able to be in the show. He was however able to be a part of the "Starship Ranger" video that played at the beginning of the show. You can watch the video here.
In AVPSY, he reprised his roles as Seamus Jillian Crockett and James Potter, including singing as James in the song Everything Ends.
Brosenthal was also a part of the Apovalyptour
Oh goodness. I was looking through google images pics of Brian and I found this hilarious one of he and Joey at a U of M football game. Joey looks like such a baby!