After looking at the EF and RF mount carefully, I have a difficult time envisioning modification to the mount that would retain adequate strength and line up the contacts. It might be possible to modify the mount to accept both, but I don't see it as practical, perhaps some sort of design that rotated mounting slots, but a Rube Goldberg mount is not for a 1 series camera.
As far as diameters, both EF and RF are the same. The best way might be to mount a RF lens upside down so it matched with a 2nd set of contacts. The question is having adequate mounting strength with a dual mounting setup.
If a mount can be designed, then a collapsible mount or moving sensor would work.
There may be such a beast out there as a prototype, but as to actually producing it??
In any event, the physical design of the camera is locked down by now, and production of the pieces and parts well underway.