So relative to file ownership and such, how would such a feature be implemented at the unix directory level?
Is the "easy" implementability of the feature related to the differences in the file system implementations between Windows and Unix?
One interesting aspect of this is that the feature would have to implement some amount of "permissioning" between two different users, depending on the implementaion. I mean one could either implement it as creating a directory over in the "other" users home directory and then automatically create a symbolic link to that directory. In this case you do need to sudo to the "other" user to create the directory and then sudo to the original domain directory owner to create the symbolic link.
In the other implementation you'd somehow actually create a file in the original web directory that is owned by the "other' user. This either requires root to create the directory and chown it to the new user or requires someone temporarily change the directory permission to allow group access (assuming the other user is in the same group) and then have the sudo to the other user to create the directory. This implementation is somewhat problematic because the original user now has this directory that they can't move, modify, or delete in their web directory.
And even if this directory is created, it's not clear to me that it could be ftp or webftp accessible to the "other" user.
It's past my bedtime so I apologize if any of this doesn't make total sense, but I hope you get the idea that the implementation of this feature under the system configuration at Dreamhost and using the unix filesystem permissions needs to be carefully thought out and even then may not do exactly what you want.
In your other hosts implementation, how were the filesystem permissions handled? What did shell users see?