I don't think these Leica M comparisons are valid at all. They will make an autofocusing 28mm f/2 Summicron-SL for the L-system, and just look at the size of that one (before we even get to the price)
As much as I like my Leica 35/1.4ASPH on the R, I agree we can't expect an AF lens that small and I'm sure Canon won't make MF lenses. I guess it's academic though: I can just use Leica M lenses now; that suits my needs fine.
The lowest one can expect should be somewhat similar to the Sony FE 28mm f2 (has to be bigger with the control ring, etc.) but Canon is probably not interested in making one like that. If size was their main priority over rigidity and optical quality, they would have chosen a smaller mount and an even shorter flange distance.
With all that said, there will be at least one pancake lens, it just might not be what people expect (you probably cannot make a 28mm as small as let's say a 40mm).
My happy lens would be the size of the EF 35/2 (43mm thick) or 50/1.8 (39mm), or just a bit shorter. The right-hand grip already stands out 35mm or so, so a lens up to 35mm makes the camera with lens mounted no thicker/deeper than it is already. This is achievable for the R mount.
If size was their main priority over rigidity and optical quality, they would have chosen a smaller mount and an even shorter flange distance.
The Canon M system is doing that job. However, people who use the big Canon bodies use them for more than one thing. I'm happy to use monster lenses for most work. Some work needs smaller lenses though, and I'd like to have my R with me everywhere just like I had my EOS M with me everywhere (and Contax G2, and Yashica T5 before that).
Given that the body height and width is bigger than the mount diameter, no, the mount diameter doesn't constrain portability. And a shorter flange distance would make the camera smaller but lens bigger.
The REAL criteria is: is the lens longer (deeper) than the handgrip? If not, then the lens isn't what's determining the total dimensions of the camera. Whether it's a pancake lens, or 35mm thick, the camera thickness with lens mounted is the same.
With all that said, there will be at least one pancake lens, it just might not be what people expect (you probably cannot make a 28mm as small as let's say a 40mm).
Well first, the EF 24mm f/2.8 pancake IS as small as the EF 40mm f/2.8, so the lens clearly can be made that small. Secondly, that 24mm is a retro-focus design and a recomputation for a rangefinder/mirrorless would be even smaller. A 28mm would be in between the size of these two lenses but closer to the 24mm.
But thirdly and most importantly, if you have the f/2.8 zooms with you then a pancake 2.8 is useless. It brings nothing to the table. In contrast, a modestly-sized 35/2 or 50/1.8 brings an extra stop or more.