Interesting idear. Now one way to accomplish this would be to use a 6x9 camera that also has advance windows for 6x4.5. Using the camera without the 645 mask, but only advancing the film for 645. Everyone would shoot at double the film speed (i.e. shoot at 400 for an ISO 200 B&W film).
Not a round 6 project but lets keep it on the table for a special LAS1C round.
Anybody got one of those old Bessas without the 645 mask?
tim in san jose