It would be great if you can work on this one!
TBH there's no good support for breadcrumbs atm in Jenkins. They are just generated iterating through request ancestors, something that I guess is provided by Stapler (https://github.com/jenkinsci/jenkins/blob/07c48b03fb4c651e7314e7adbe3a0ef0fe249586/core/src/main/resources/lib/layout/breadcrumbBar.jelly#L44).
A hacky way to do it that comes to mind could be to check if the ancestor points to the rootURL, then it could be replaced.
That said, I'd favour using some Java code to do this. If we get a Breadcrumbs module going on, then we can expand it in the future with some utilities like:
- Get the last breadcrumb item.
- Check if a breadcrumb item is the current one (good for styling purposes).
- Prepend/append items to the breadcrumbs.