If you are in a connector rule or a before provisioning rule, you have access to the application (typically). Please see the Rule guide on compass for the particular rule you are checking.
If you do have application, you can access the value using the following
Object baseUrl = application.getAttributeValue("genericWebServiceBaseUrl");
This can access any object available on the application, regardless of type.
Please note: this is currently available, but as Rules get replaced by OOTB functionality this may not be available in the future.