Thank you very much for sharing your use case. That is indeed a very interesting scenario.
Makes sense.
The previous implementation of LAMBDA_FORWARD_URL sent a POST request to the given URL with the lambda-function-name in the header and input event as JSON body. I guess that should be sufficient to cover the mockserver scenario or are there any special requirements to consider?
I added the feature to our internal Lambda backlog. We continuously monitor popular 
GitHub feature requests and Discuss posts when prioritizing our efforts to improve LocalStack.
Best,
Joel