Hello Graham
Based on your requirement is it not possible to REDEFINE the service? Service Redefinition can be useful to put more logic/condition.
Gateway also provides Service Extensibility concept from SP9. Check out this https://scn.sap.com/community/gateway/blog/2014/08/08/extending-a-service-using-the-gateway-service-builder
In this case the inherited service overrides the base service, that meas the using the base service URL you can call the Redefined service.
You right, these configuration tables are filled by Gateway Registration mechanism. If you try to do so, you may explore transaction /IWBEP/REG_SERVICE and /IWBEP/REG_MODEL. Here you can change the model and service by providing new MPC/DPC class, which contains your new functionality.
Regards
atanu
