Can I maintain a modified version or vendor branch of third-party software using Subversion?

Yes, you can maintain a vendor branch using Subversion. Subversion can be used to track local changes to third-party code, even across upgrades from the third party - that is, you can maintain your own divergent branch (vendor branch), while still incorporating new releases from the source.

If the vendor code is hosted in a remote Subversion repository, then you can use Piston to manage your copy of the vendor's code.

As a last resort, if using svn_load_dirs.pl is taking too much time, or if you're looking for an easier solution, see Jon Stevens' step-by-step explanation at Subversion Vendor Branches Howto. This solution does not make use of the space-saving features in the Subversion backend when you copy new code over old code; in this solution, each import of a vendor code gets an entire new copy, and there is no space saving for identical files.