Unfortunately I don't have anything code-wise. I once started to separate the ResourceResolverFactory and ResourceResolver part from the JCR part but I didn't come very far (and I fear I lost my work)...
The basic idea, really is to split the JCR Resource bundle in a ResourceResolverFactory bundle and a JCR Resource Provider bundle (also providing the JCR Event to OSGi Event bridge)
The Domain Mapping functionality would be part of the ResourceResolverFactory bundle but would probably have a new exposed service API to plug into it. Such that for example Vanity URL functionality could in fact be factored out into another bundle.
Sounds good. Maybe we also get to solve SLING-2076  as a side-effect :)
And yes, this would definitely be something I would be interested in.
Could this work be started in a whiteboard location ?