The source code for FogBugz appears to be available in its installation directory, along with its compiler and build script. I was able to make LDAP work for community users by commenting a single line. Community users logged in via LDAP still have restricted rights, so I don't think it defeats the spirit of licensing.
I'll leave it up to a FogBugz employee to make the change or reveal the details, as I don't feel it's my place. There could be side effects or other implications of which I'm unaware.