Allow read-only observers to contribute to a repository using only pull requests

I'd like to see a way for read-only observers of a repository to be able to clone the repository, make code changes, and submit a pull request for those changes.

In Github, the workflow would be to fork the repository, then push a new branch, and submit a pull request from that fork.

It would be nice to see another way to contribute without the overhead of a fork. One way may be to allow observers to push branches called "pullrequest/name". Pull requests could be created from those branches but they wouldn't have access to any other branch.

Another option would be to accept a Git Patch as a pull request. Rather than requiring a branch to merge, submitting a patch file against a branch could create a pull request.