I'm testing a new release of IXR, which includes some important bug fixes and merges in most of the changes made to the WordPress fork. The changes I've skipped are the optional vendor arguments to IXR_Client (which isn't used by the WordPress source anyway) and the timezone support - everything else is in there.
It would be great if WordPress could use this version; if not, you'll need to merge in the bug fixes. I've tested the new version myself but I'd strongly recommend testing it further before rolling it in to a full WordPress release.
The new version is available here: http://scripts.incutio.com/xmlrpc/tests/IXR_Library.inc.php.txt