There should be an option by which the normal suppression of Multibranch projects for origin PRs would be disabled, so you would get two projects:
- one building the head of the branch
- one building the speculative merge commit.
There could also be an option to suppress the origin branch project when a PR is filed from the branch. This would cause the origin branch project to be deleted once the PR is filed. (It could be resurrected if the PR is closed but the branch not deleted.) This option would necessitate a new API in github-branch-source to filter branches.