The patch is hard coded! You will have to use an installation without the Project Cart. patch.
Try this:
1. Uninstall Halo 2,
2. Goto Program Files/Microsoft Games, yatayatayata, delete the Halo 2 dir.
3. Reinstall Halo 2.
4. Launch to verify it works, seriously!
5. Then this is the kudegra! In the Program Files/Microsoft Games dir, make a copy of the Halo 2 dir and its contents into the Microsoft Games dir.
6. One can be patch with the Project, the other will use GFWL.
7. Create necessary shortcuts to launchers.
See i am totally genius!