Sorry for getting back to this post so late.
The way I got it to work was this.
I installed phpMyAdmin in /home/phpmyadmin
Then in Webmin -> Apache Webserver -> (Choose a Virtual Server)
Then, under "Create Per-Directory, Files or Location Options"
Type: Directory
Regexp?: Exact Match
Path: /home/phpmyadmin
Click the Create button. On the page that comes up (Per-Directory Options), make the settings for ALL the categories match those for /home/(account name)/public_html (from the previous page).
Then, back on the "Virtual Server Options" page, make an entry in Aliases and Redirects:
From: /phpmyadmin
To: /home/phpmyadmin
Once that's done, users can go to
http://www.whatever-domain.whatever-extension/phpmyadmin and will get the login page for phpMyAdmin.
Hope that helped.