Ok, this question got lengthy, but I wanted to give all details, actual question is in bold below.
I have defined an External content type, let's call it ExternalCustomer.
I use an ExternalCustomer-column in a doclib, so user can select an external customer to his document in a picker dialog.
I have used SPD to create the ECT and selected my WCF service method as my default reader method. Since there will be a huge number of External Customers I need to send an extra parameter to the method, a guid that is stored on the specific SPWeb where the doclib resides (and can be in the URL as well if needed).
The service signature looks like this
I want to change it to
List<ExternalCustomer> GetExternalCustomers(Guid regionId)
and somehow sneak in that extra parameter somewhere so it reaches my service. I'm creating the docLib in code from a custom listdef during init of the SPWeb. I can edit the XML that defines the BDC.
I don't want user to input the Id (as a filter parameter), I would prefer to not get all customers and then filter 99% of them away.
(As a backup will have to define a custom field type with a custom picker, but i have more then one external content type...It seems very much work for this tiny extra functionality)
I run SP 2013, I have both the service provider and SP on my own servers.