We once visited when BTMRR was coming out of refurb, and for whatever reason (perhaps a cushion in case the refurb went over) they weren't releasing Fastpasses for it for a few days after the refurb was scheduled to end. I tried at 60 days and thereafter, and it would always show "no availability." The day BTMRR reopened, however, Fastpasses magically became available, and I was able to hop on the app and get a day-of FP for it from another park (after we'd used our 3 pre-reserved FP for the morning) as our 4th FP for the day.
In other words, if they don't make FP for it available ahead of time, be sure to look for them the day-of when it reopens. It might make a nice 4th-or-later FP choice.
