I think it is very important to have unified way to get all the different permutations of ground truth. Prior to these - exciting - new developments, there was one place to find the ground truth - in solrconfig.xml. You could grep the file and get everything you needed (well, except for admin ones, but they were mentioned in the comments).
But now, we have a lot more handlers that are implicit plus the hardcoded ones and the overlay ones. If there is no way to get all of the information (including the source of the definition) in one place, there will be confusion down the line. From users and - I suspect - tool writers.
So, I think a single end-point with parameters to differentiate the source would be nice. The parameters could be ?source=all, source=implicit, source=defined, source=overlay as well as status=active, status=overriden and status=disabled/inactive (maybe). By default, it would be source=all and status=active. These two parameters would be included for each entry.
So, if I have /selectAR in solrconfig.xml and it's also overriden in the overlay then it will be returned as source=overlay, status=active when I request with source=all, but as source=defined, status=overriden when I request with source=defined.
Does it make sense?