I think the most sensible thing would be to gate registration by a workflow that performs the same checks by the registry: General/.github/workflows/automerge.yml at master · JuliaRegistries/General · GitHub
Therefore, a release-please PR would only be merged after the registration CI is happy.
Which is probably what we ought to be doing anyways, rather than relying on the public registry backstop CI as the only validator. (And even when the registry does catch some issues, it still requires an awkward PR to fix the compat entries in main for an already updated Project.toml)