Canon has a history of including capabilities in their new models (especially lenses) whose capabilities may not be realised for years afterwards when fully functional bodies are released. It is not beyond possibility that even EF lenses may have options for some form of dual-IBIS compatability. I am not saying the knew at the time precisely how it would work, but left things open.
Also, as far as I am aware, an IBIS camera body (on Panasonic and Olympus, anyway) simply needs to know the focal length of the camera so it can calculate the amount of correction and on those bodies if you are using a non-AF lens you manually enter the focal length and the camera does the rest. That should not be difficult to emulate in Canon bodies.
Also, for lenses with in-lens IS, some Panasonic bodies the dual IS works by letting the lens IS do the donkeywork and the IBIS just perfects it by adding correction for translational movements. So there is precedent.