Post commit update returns "Skipped '.'" with non-serveradmin user

I have a post-commit hook that cds into my website's directory and runs svn update. For my serveradmin, it works correctly. For other users, the commit works and the hook executes, but the update returns "Skipped '.'" to the log file.

Here's as much pertinent information as I can think of:

• It's a Linux environment on a Media Temple gridserver (gs)
• The repository was set up in /home/XXXXX/data/svn/reponame
• The working copy was checked out into /home/XXXXX/domains/mydomain.com/projectname
• serveradmin and otheruser both belong to the same group (we'll call it usergroup)
• all files involved have ownership of serveradmin.usergroup
• all working copy files (including .svn files) have rwx permissions for user and group
• all files in the hooks directory have rwx group permissions for user and group
• each user checks out the repo via svn+ssh, and I validated that the commit hook is executing as the user who is doing the commit (not apache or svnserve)

Tying up this old question since, as I mentioned in my comment before, this ended up being a limitation with Media Temple and their support team confirmed that there is no workaround.