You can create custom scripts, filters, assertions and roll-your-own classes that can be executed in test cases, but before we go down that path, can you explain more about the requirements you want the test case to handle?
Data Protocol Handlers have very specific implementations in the context of Virtual Services. DPHs implement overrides to framework methods that deal with incoming request and / or outgoing response objects. These request/response objects are not typically available in Test Cases which reduces what a DPH can do.