From my experience, I concur: totally impossible to do that automatically.
The only thing we could do quite easily is have that list. It woud require cloning every single repos (well, pom.xml should be enough), and run mvn help:effective-pom on those and grep maven.jenkins-ci.org in there. We would then have a list of to-be-updated plugins. Then we could decide to try and fix the most important/installed or so.
At least this easy step would give us data to plan for the changes/PR per-se.