How can I extend the HttpConnector class with access restriction

The HttpConnector is internal and not intended to be extended. But you can create your own connecter and let it implement IHttpConnector. You then publish it using the "com.ibm.team.workitem.service.dataConnector" extension point and can use it by its ID out of your filtered value set provider (either a script of a deployed Java class).

We consider this as an very advanced way of writing you filtered value provider. Our hope is that the provided HTTPFiteredValueSetProvider is rich enough to cover most use cases.