How hard of a process would it be to attend Sapper School as a 91B?

In the USAR as a 91B? Just about impossible unless you're in an engineer unit

Edit: I missed that you are in the Reserve so yeah not too likely.

In 2015 they closed it to other MOSs when they made the BEBs and needed to get more 12Bs through. But this is on the Go Army page right now:

https://www.goarmy.com/soldier-life/being-a-soldier/ongoing-training/specialized-schools/sapper-leader-course.html

"To wear the Sapper Tab, a Soldier must be a Graduate of the Sapper Leader Course which is operated by the U.S. Army Engineer School at Fort Leonard Wood, Missouri. The Sapper Leader Course is a 28-day course designed to train joint-service leaders in small unit tactics, leadership skills, and tactics required to perform as part of a combined arms team. The course is open to enlisted Soldiers in the grades of E-4 (in the Army, specialist) and above, cadets, and officers O-3 (Army, captain) and below. As students can come from any combat or combat support branch of the service, female soldiers are permitted to attend, but priority is given to engineering, cavalry, and infantry soldiers. The course is divided into two Phases..."

So priority is to EN, CAV and IN - and after that if there's any slots and you're not those MOSs you can go. But I don't know how often they are letting in other MOSs right now. And you have to be E4 or higher as enlisted.

In my 10 years in the reserves I never met a sapper that earned the tab as a reservist. Its hard just getting air assault and airborne slots let alone sapper. Unless you're in an airborne unit that is.