I've been using a custom scanner to gain access to OSGi bundle via classloader. Previously (4.x) I could do this via PersistenceUnitDescriptor, but due to the decoupling, also this feature has now been hidden from me.
I don't mind tight coupling myself so things that would make my life easier would be.
Introduction of a buildArchiveContext extension point so that I could override to customize ArchiveContext
Public access to environment, options, and parameters of ScanResultCollector.
Public access to persistenceUnitDescriptor of StandardJpaScanEnvironmentImpl (either directly or via a new intermediate interface)