Using github merge queues with self-hosted buildbot CI

Question

We've been using bors-ng for our merge queues for some years and now we'd like to migrate to Github Merge Queues.

I find GitHub's documentation difficult to follow, but as far as I can see, I need to do the following:

1: Set up buildbot to build merge queue branches:

e.g.

c['schedulers'].append(schedulers.SingleBranchScheduler(
                       name="simple-ghmq-branches-scheduler",
                       change_filter=util.ChangeFilter(
                           branch_re=re.compile("^gh-readonly-queue/"),
                            repository=REPOS,
                        ),
                        builderNames=["buildbot-build-script"],
                        treeStableTimer=TREE_STABLE_TIMEOUT))

This is very similar to what we had for our working bors-ng setup (just branch_re=... was branch=['trying', 'staging'])

2: Turn on merge queues in GH

Add a branch protection rule for the main branch and turn on merge queues:

3: Add buildbot as a required check to the base branch

You can only add required checks that recently notified github. I just wrote a little python script to do this.

After that, in the main branch protection rules, set buildbot as the required check.

4: Add a workflow to the main branch.

As per these docs.

e.g. in .github/workflows/ci.yml:

on:
  pull_request:
  merge_group:

5: Raise a PR

Click the "merge when ready" button.

But the problem is, github never makes the branch for the CI to pick up, so it just sits there forever.

Can anyone see what the problem is?

EDIT: also tried removing the required status checks from main and adding them to a new branch protection rule for gh-readonly-queue/*. Under this setup, github did make a branch, and buildbot did build it. Buildbot reported failure, but github still merged the branch... I don't get it.