Hello Jakob,
in the NetWeaver Addon we have implemented a generic POWL-OData service that can expose any POWL as an OData service. No coding is required, only some customizing. You will find the documentation in SAP Help under the following link.
http://help.sap.com/nw-uiaddon#section1
The UI Addon is valid for NW releases 7.0, 7.01, 7.02 and 7.31. As of release 7.40 (which supports HANA) there is no addon anymore but software component UI_Addon. The POWL functionality can still be used. In addition we have created an OData Cache for storing the entity sets and thus boost the performance of the POWL OData service. Information about it you can also find in the aforementioned link.
Regards,
Otto Makris