With the coming of modular Xorg I think it could be useful to add an "xcomposite" [global] use flag for packages that can support such an extension (e.g. metacity and xfwm4) like what has been done with xinerama and xv.
Please file a specific bug for each app that supports composite, and make them block this. They can be assigned to the maintainers of their apps.
Again, please file bugs for each app separately.