From what I have read if you are going to use the open source subversion server, your options are limited to modifying the access file as you were hesitant to do.
The issue is mentioned here, although in regards to LDAP auth: https://serverfault.com/questions/188023/webinterface-for-configuring-svn-access-in-mod-dav-svn
My advice is to set up a cron job to automatically generate the auth file on a regular interval.