Lockable resource do not solve this issue. Let me clarify my use case:
I have a shared Jenkins installation. We have a number of teams using multi-branch pipeline jobs. Sometimes a team will have a large number of branches build concurrently - for example, they have a lot of feature branches that all depend on a given snapshot, and are configured to build when an upstream project builds. Another example is when a project is migrated from SVN to git. Regardless of the actual trigger, the result is that every other project is effectively blocked out because we have no realistic way to limit the concurrent branches for a given project. Implementing a lockable resource for every single project could technically solve the issue, but would be overly burdensome to administrate, and would then restrict us to a single concurrent branch, where we would prefer to allow up to four concurrent branches per project.