If they just go to 12 to get a conference championship game/divisions, I think they will add two all-sports members to avoid looking too "mickey mouse". Adding BYU and BSU as football-only would be part of a bigger expansion that also added two all-sports members (Cinci and Memphis, for example).
I think done in this context football-only would look innovative, rather than a shortcut. Just my opinion though. Also, count me in the crowd who would prefer to see BYU in the B12 as football-only over all sports. The WCC is a great fit for BYU.