(Ryan:) I have through around this a bit and I have been thinking of something even simpler. Just proxying requests with a specific URL pattern (say /api/providerregistry/*) and sending them to a specific configured domain service (so in this example the provider registry). So basically just acting as a proxy to the domain services.

(Carl:) While URL pattern matching would certainly work for me, and while I don't have any particular preference, I do think it is worthwhile to look at existing standards for making all these services known and to keep the software components as "pluggable" as possible.

I know WSDL has a bit extra overhead, but it may be worthwhile as consideration.  Perhaps it's time for a spreadsheet to compare the pros/cons of different options e.g: