In Germany we have Frankfurt-Egelsbach, EDFE, which is a GA INFO airfield within the Frankfurt, FRA, "D" controlzone. It has it's own approach and departure proceedures which were worked out by the Frankfurt ATC unit and mother company, DFS, and is coordinated with Frankfurt ATC. In normal, everyday flying you never need to contact Frankfurt, you contact EDFE directly. The system has been working for some years now with very few problems. It doesn't (yet) have any IFR.
As far as I know, other GA fields within "D" airspace, like Paderborn-Höxter, are controled by the local ATC, but I'm not sure on that.