I use Nikon so most of my lenses have a 52mm filter thread, which is nice.
I was thinking maybe an inexpensive rangefinder would make a decent camera to dedicate to IR since I wouldn't need a meter. I didn't think about a TLR though, that's a good idea. I would want one with interchangeable lenses, preferably with an ultrawide option.
I had never seen the Mamiya 7 polarizer; I googled it and that's almost exactly what I had in mind! Tiffen or Hoya should copy that idea to make an IR filter for SLRs; just swing it out of the way until you're ready to take the shot. I guess there might not be much of a market though.