I'm asking this for a coworker, I think it would be better, if he has a patch for an open source project to just send it to the admin of the project, but he wants an immediate solution and I don't want to convert an external repository.
So, is there any way to set up SVN to patch a file in an external repository when you check it out or update the repository?
No. It's not possible to run a hook on checkout or update... so svn isn't going to be able to directly apply your patch.
I know you mentioned you don't want to convert your svn:external to an internal repo... However I'd most likely to go that route. The red book has a solid article on vendor branches. Hopefully the patch your coworker submits will be in the next release of the library, so you won't incur a cost updating to that new release and reapplying your patch - you could just go back to an svn:external at that point.
If that's not workable, other things that come to mind: having the build script apply the patch, or patch & build the library and publish it to an artifact repository (nexus or archiva) and having the team resolve the dependency from there.
