In a public, decentralized wiki, that would certainly
be a problem.
In a corporate environment (where this is be needed most) though, it
would "only" be cumbersome.
Yes, it would probably be doable in a corporate environment, however I
imagine there are better ways of doing it than a wiki with a
cumbersome encryption system hacked on. Is there any great need for
the restricted pages to be alongside the unrestricted ones? Put them
on a password protected server and be done with it.