Upload & Sell: On
An ~20mm pancake prime has been done several times in the past (e.g. Olympus Zuiko 21/3.5 and Voigtlander 20/3.5 SL-II are quite compact), so this is quite possible. A 100mm lens several times shorter than its focal length would be more an (unlikely) unprecedented accomplishment. Olympus made a rather compact 100/2.8, but "rather compact" in this case means almost as small as a typical 50mm lens, not pancake-sized.
I'm thinking of something for a full frame Canon DSLR. So, Gochugogi already has an explanation for it.
The Oly 21/3.5 and CV 20/3.5 SL-II that I referenced are both FF lenses (that can be adapted or are native mount for Canon EF), so pancakes in this range are definitely possible (though pushing up against design limitations for the reasons Gochugogi mentioned). A 14mm lens along these lines might just be possible for EF-s (assuming mirror clearance is scaled proportionally with the 1.6x crop --- what's the actual mirror clearance on EF-s?), though not for FF.
Any chance of a 400/2.8 pancake?
Only if you're hungry for a giant, all-you-can-eat pancakes stack