GitHub heralded one fantastic thing that didn't exist before - easy forking or a repo. That is more likely to force an 'upstream' dev team to be more benevolent to directions of others. Perhaps because of the possibility of mind-share shifting to the fork and away from the upstream/origin. That divergence from the origin happens isn't a problem necessarily. I've been doing open source for 18 years now, and the only thorny thing that remains is trademarks (were the forker wanting to release a binary) and a lesser honor system: "hey stop using the name of my OSS tool/lib because you're just confusing people". Node.js and Io.js is a notable case <https://www.theregister.co.uk/2015/09/09/node_js_v400_reunites_with_io_js/> .
If you have your patches, then just apply them to a fork on GitHub. You don't have to become maintainer - make that clear in the readme <https://svn.apache.org/repos/asf/xerces/c/trunk/README> (oooh maybe http://makeareadme.com/ applies). Start something there and see if there are others that want to help. One of them might become maintainer :)
