I have an RZ. I use it to shoot IR landscapes, which is not hard because you can compose on a tripod then add the filter and shoot. I would like to shoot handheld at larger apertures without huge flashes, e.g. for portraits, but it's impossible to focus due to the small amount of visible light that comes through. The film is sensitive enough that I could shoot in sunny-16 at f/2.8-f/4 and 1/60-1/125.

Has anyone tried modifying a medium format back with a sheet of IR-pass gel over the film gate, e.g. a Wratten 89B or Cokin equivalent? It seems to me that you'd have an unobstructed viewfinder image and record the scene in IR but I don't know how thick the filters are and whether they'll cause a focus offset.

All the Cokins I've seen are on the order of 3mm of resin, which seems pretty significant to me, even if the refractive index is lower than glass. You see plenty of people modifying digital P&Ses for IR (remove the hot filter, replace with plain glass or R72) and thickness to about 0.1mm is critical there. Admittedly, the focal lengths they deal with are about 10x shorter, so I've probably got 1mm of leeway.

Anyone had any experience with this sort of thing? If it's something that I can be confident of working, I'd happily modify a dedicated back for this purpose. Conversely I don't want to fork out for a P007 if it's never going to look good.

(No, I don't want to buy a TLR just for this purpose)