Mark the roll, eh? A mark that I should look at, with the lights on? Or that I can't see in the dark with my fingers near a sawblade? Can't say I'm liking either option, and an RZ back is not going to deal well with film that is the wrong width and burrs all down one edge. It also means cutting one end off the metal spool that the film is held on, so there will be little metal burrs spread all through the film, not to mention stress-fogging.
Far easier to put a razor or two in a little channel/guide and pull the film through it as it's travelling from the large roll to the small. There are a number of film-slitting designs on the web and they're pretty simple; I could make one up to slit the film as it came off the 70mm roll.