Can a single WCF service offer multiple interfaces, and if so how would you express this in app.config?
I mean one services offering several Interfaces
The following looks closer to the original goal and doesn't involve one large interface...
Multiple Endpoints at a Single ListenUri: http://msdn.microsoft.com/en-us/library/aa395210.aspx
The sample linked to above explains that it's possible to have multiple endpoints registered at the same physical address (listenUri), each implementing a different interface (contract), e.g.:
This is possible because incoming messages are routed to the appropriate endpoint based on a combination of address and contract filters.