As part of my cvs.fusebox.org project, I made up a little CFC that will perform authentication against a CVS passwd file. I didn't want to have to maintain two sets of credentials for all the CVS users, so this proved a very good solution. The implementation was actually pretty simple; the most difficult part being the crypt script (I used Perl) for doing the UNIX crypt on the passwords.
Because the passwd format is pretty standardized, the CFC should work on .htpasswd or even /etc/passwd files as well, as long as you're not using shadow passwords. I haven't packaged it up for distribution, but if anyone's interested, I'd be happy to send you a copy, along with the Perl script.
Any particular reasons to not use Subversion?