Ok, so this is going to sound mad crazy (did legs in the gym today, so my blood isn't where it should be at the moment), but would it at all be possible to say, have 2 4x5 lenses, one that I would use exclusively with the 3.25x4.25 instant film, and one that I would use with 4x5 film, with the trick being that the crop factor would be compensated for between the two lenses, e.g. if the instant film was 1/2 the diagonal of the 4x5 film, get my lighting figured out on instant film using a 105mm lens and then I could do my actual shot with a 210mm lens? I could also use camera movements to "center" the frame for the instant film (not physically move the frame, just shift the composition so that it matches up exactly)?
Even if that did work, this is going to take a LONG time (take a shot, wait for the film to develop, adjust lights, repeat, change lens, tweak composition, check focus, expose), and I was already concerned about how long it would take to do portraits with 4x5.
Maybe I should just accept the heart crushing blow (thanks Fuji!) and get a Mamiya RZ67 to do my portraits.
@wildbill: As for the option to use digital - I have a very good DSLR that I use all the time, but I don't want to go down that road in this instance. It is a good and valid option - and there is certainly nothing wrong with it, just isn't what I am looking to do, but thanks for the suggestion!