I know that Steve Young used to come around once in a while and throw with the receivers (current and alumni). As I remember, Young also has a good relationship with Doman. I'm assuming he tries to keep a low profile just because he doesn't want to overshadow the current players and undermine any coaching that is going on.
I'd like to see BYU keep a good relationship with Sarkisian because he's both competent and high profile. Plus BYU and USC play a lot of the same opponents, so a little bit of inter-staff dialog wouldn't hurt.