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Since there's no way to effectively mount the Canon lens to the tripod and just slide the back (camera), you have to do a little dance with the RRS clamp to accomplish that. You do both - slide the camera AND shift - simultaneously. The RRS clamp already has marks scribed at 12mm left and right of the center, so you start with your camera centered and lens centered, then shift left and slide right, then shoot with everything centered, and finally shift right and slide left 12mm. The effect is that even with the shift, the lens stays in the same spot, eliminating any parallax that would have been caused by simply moving the lens. It's a convoluted way of doing a rear shift on a view or technical camera, and while a bit awkward, is quite effective. Until I (or someone else) comes up with a tripod clamp for the lens, this is going to have to be the solution.
Could this be what you're thinking of?
Canon TSE Tripod Collar