We should run the phan CI job unconditionally for all extensions to improve developer productivity and encourage the use of phan across all our codebases.
The job would check if .phan/config.php exists, if not, it would return 0 and quickly finish. When someone wants to add phan to an extension, as soon as they add the file, phan begins running.
This would consume a small amount of CI resources to start a jenkins job, clone a repo, and then do nothing with it. However CI resources are pretty cheap compared to constantly running "check experimental" and then waiting for a CI team member to merge your patch to run phan jobs.
This would not help in the case of extensions with dependencies, but it should reduce the barrier to using phan considerably IMO.