Rangefinders are best with IR
Since the viewfinder is independent of the lens, you could focus and compose easily even when a rather dense, almost opaque filter is over the lens. Deep red filters almost make it difficult to use reflex viewing except in the brightest situations. Using a filter like the IR-72 would make the SLR finder really go blind since the filter is almost totally opaque.
I've used High Speed IR on my Leica M3 without problems of fogging or leaks.
The last time I had IR film (Ektachrome IR), it went into a bottom-loading Zorki-1. I had to trim the leader in a changing bag and load the camera in it as well. Not really harder than loading the same in other cameras since a changing bag is really required. Both cameras have cloth shutters. The Zorki even had a home-made shutter curtain, and it proved rather opaque even to IR rays.
Jay