You are absolutely right. Never have seen adapter like this. But it would be possible to design one and make on printer, plastic of course...For what? I don't know, maybe to prove it is possible, or for some kind of display? Sorry, I know better then...
This is actually not true. It is absolutely possible to install RF lens on EF mount body as well as EF-S on EF. But of course you won't be able to use it in such configuration. Just not enough room to fire mirror. This may be the reason that adapters...
