Hi Jesper,
have you actually tested this?
I just did with the configuration you proposed. I used an app account with r perms to the repo root to setup the repo in sventon. Then, configured the access of my own user account according to your proposal below and I confirmed that I can 'svn up' a WC of a checked-out subfolder, whereas checking out the root is denied. When I went back to sventon to access 
https://svn.domain.net/sventon/repos/testrepo/list/myproject/ with my user credentials, I receive:
 
Authentication required for directory: myproject/
Authentication failed!
Thx much,
-ar
PS: I got an error when attempting to reply out of google groups site.