I wrote this to the PyXPCOM list, but then thought I'd blog it for those who don't read that list:
A while back we open sourced the bulk of Komodo Edit, which includes our build system for Mozilla, which builds and handles lots of edge cases with PyXPCOM, including a series of patches we have to build, fix or add functionality. Komodo uses this build system and these patches, and has proven to be stable with PyXPCOM over a long period.
I wrote an email about it to the openkomodo-dev list a few days back describing the basic information for building XULRunner with PyXPCOM. With just a bit more work, the build system could probably build a redistributable XULRunner + PyXPCOM. While we (ActiveState) wont have time in the immediate future to do that work ourselves, I thought I'd mention it in case anyone was interested in digging in.