This is a problem that has been discussed earlier (see bug 230). According to the FSSTND, it should be possible to mount /usr read-only. There are several advantages of doing that. I expect to be able to configure makewhatis to create /var/catman/.../index.* instead of /usr/man/.../whatis. (Please no discussion about the FSSTND.)
It uses /var/cache/man in 7.0
Created attachment 153238 [details] Bug 8653