The flange distance for RF is 20mm so making some form of adapter should be doable, although I'd note that the TS-E17+1.4x gives ~24mm with ±17mm of shiftHow are you mounting your M645 lenses to the R/RP?
I might take a trip to the Photography Show this year (for the 1st time in a very long time) to have a look at my options. A Cambo Actus G together with something like a RODENSTOCK Aperture Stop HR Digaron S 23mm lens would be my dream. With a 70mm image circle it would be great with a mirrorless body however the flange distance on this lens is only 44mm so not sure if it would work?
But if Canon eventually release a high MP body and update the 16/24mm TS-E and add those nice auto correction features it would make kit like the Cambo/Rodenstocks less appealing.
I do (some of) this stuff for a living, so am regularly reminded that if Karen (my wife/business manager) can't see the difference, then it's good enough for clients ;-)
As to M645, see this article...
Review of the Fotodiox TLT ROKR tilt-shift lens adapter for mounting medium format lenses on mirrorless cameras. M645 to Canon RF and M645 to Nikon Z
Here's a 210mm f/4 with 15mm of shift and 10º of tilt