I shot the mamiya 330 for years, and never found a reasonable way to do what you are thinking about, the only thing I ever did, was when I wanted to change rolls, was run it through the rest of the exposures, then remove and load anothe roll of film in the emulsion that I wanted to shoot, yes, I ended up with a bit of wasted film, but it was a cheap price to pay to get the shots I was able to get. If changing mid-roll is something you want to do regularly, then as David suggested, your best to get a camera with a removable back, which are surprisingly reasonable now a days.