Suppose I have multiple instances of some line of business app in my environment. Is there a way to create a single External Content Type that can be parametrized and reused? If I have 5 instances, is it necessary to create 5 external lists and 5 external content types, or can the lists indicate which data source should be used?
I think the best approach in this case would be to do the CRUD routing at the web-service/stored procedure level.
Alternatively you could write the models in Visual Studio and build logic into the CRUD methods to determine where the record should end up.
Depends where you feel more comfortable placing your business logic, if this is going to be common practice then I would say webservice.