Yes, I have been using fp100c in a holder designed for polaroid pack film. No problems. Very nice stuff. Seems to perhaps have better shelf life than the polaroid, in my experience. I mean, I have year-old fp that I lost in my move and just found it, and it shows no colour shift.
The 4x5 fp100c is *way too good* IMHO to waste on proofing. The prints can be truly remarkable.
The biggest differences between the fp and the polaroid is that the emulsion of the Fuji stuff is superduper strong, you can throw your print in boiling water and pull off the emulsion and work it quite clumsily with your hands, it won't break as easily as the poalroid stuff. Then you can let it dry and glue it down or whatever. Keep that in mind for a rainy day! So emulsion transfers are fun with the fp. On the other hand, I've never gotten an image transfer to work with the fp. Somebody here did, but it was a really funky procedure like: develop partially, then pull it apart in the dark and press on it with a moist warm rag or something.
Try it you will like it. I told Fuji about a year ago that I have a strong desire to shoot fp100c in 8x10 instead of 809 ($$$) and they wrote back and said sorry, no can do for now. Fingers crossed.