user registration service provider application

[Nate Klingenstein]

My mistake, wrong Synapse.  I'm a little surprised there are so many products with that name...

Anyway, it sounds like you want to take the attributes received from an IdP, "do something with them", then return to the SP and the resource.  That sounds like you could just use the sessionHook feature with a simple page like you describe:

https://wiki.shibboleth.net/confluence/display/SP3/ApplicationDefaults

In this case, doing something with them is basically just making that http call.

I don't think there are many examples out there because it's basically just reading environment variables and writing them to application-specific storage API's, which is both pretty simple code and not really recyclable.