SCMHead could extend Actionable, or simply have a getAction(Class) method which can call TransientActionFactory. This would be useful for defining and implementing APIs for associating information with branches without forcing callers to cast to a subtype of SCMHead. For example, for any SCMSource supporting something like pull requests, you would like to be able to define an action listing the author and so on.

To expose this kind of information directly via REST (Stapler exporting), it would suffice to make BranchJobProperty an @ExportedBean, same for Branch and SCMHead, etc.