Sorry folks, that answer is "No."
With some of Canon's EF lenses specified as not EF extender compatible, such as the Canon EF 70-300mm f/4-5.6L, the rear element moved far enough forward to provide clearance for the extender to fit, and these combinations worked fine (as long as the impact was avoided). The Canon RF 100-500mm F4.5-7.1 L IS USM Lens came with the minimum RF extender compatible focal length specified (300mm), showing Canon now taking advantage of the partial range of rear element clearance.
All of Canon's EF 70-200mm lenses are compatible with EF extenders, but the RF 70-200 was not on the RF extender compatibility list. The question on many of our minds was if the RF 70-200mm lens would indeed be compatible, at least at the long end, despite being omitted from the list.
I've tried twice (just to be absolutely sure) to mount an RF extender to the RF 70-200 at 200mm, the focal length with the most rear element clearance. The RF extender does not fit into the back of this lens.
Another great idea was to mount an RF extender to the back of a Canon Mount Adapter EF-EOS R. The idea is that all EF lenses suddenly become extender compatible. Sorry, but this answer is also "No." The RF extender does not fit into the mount adapter.
That said, the baffle physically preventing this installation appears to be plastic, and plastic is easily modifiable. I can't spare an adapter for this mod right now, but if you have a donor adapter you want me to try modifying, let me know.