I am more than willing to do the work on this design/impl, so let me know.
We can always use help! I think the next step is to find the users of custom registries and interview them to find out what there pain points are in practice. Then an RFC can document "these are the problems they are having and this is how to directly solve them". Changing the interface for custom registries is a large permanent ask. Adding an alternative to Git doubly so. An RFC is going to need really well researched justification. Building a plugin, as @gbutler suggested, and dimistarting that it is getting used, would also be a good way to demonstrate the usefulness of the design.