In my humble opinion, GitHub should bend over backwards to get this to work for him. Project admins really need better control over who can create pull requests, and what they should look like.
- create a branch or tag containing the merge so that it's easy to pull the change without having to add additional remotes.
- provide instructions on how to pull, merge and push the request saying explicitly that the merge should be inspected and tested before doing the push.
- make the latter more obvious to click than "merge pull request" button
EDIT: I'm not saying that Linus needs this, but responding to Linus' comment about it being much too easy to do poor-quality commits.
Supporting larger projects can't be a bad thing, that wouldn't be wasted effort.