FP-100B takes 30s to develop, not 1-2 minutes (FP-100C is longer, true). Switching backs takes 10s or less, especially when on a tripod. I use it outdoors, no problem. Indoors FP-3000B is even faster, 15-20s or so (I use it to proof Delta 3200 120 at times).
FP-100B is discontinued. Have you ever tried using fuji instant for proofing with real live models? I have, not a nice experience. I prefer dslr.