I've found a few things through my own research (and some ridiculously deep digging into the svnbook.red-bean link provided by Ed Marquez). Thank you for the help there.
To tell the SVN server to challenge the user, modify [RepositoryRoot]\conf\svnserve.conf
anon-access = none
auth-access = write
Now add a line in the [users] section for each username to [RepositoryRoot]\conf\passwd
bob = somepassword
The second example there works for george to have a username but a blank password.
MATLAB will now display a username/password popup when SVN challenges for credentials. This seems to persist through the MATLAB session, but is cleared when MATLAB closes.