I think the issue is that philosophically and architecturally, adding data to an incoming request invalidates the intent of the consumer with regard to the request payload and headers.
For this reason, the HTTP Current Transaction object is immutable. It seems that the VSM allows the addition of parameters, but it appears these parameters are not passed at runtime.
One workaround I can envisage is to take the incoming request, add header parameters, and send the transaction as a REST or Webservice Call. Then, marshal the response from the live system back into the lisa_vse_response list.
Perhaps, an idea should be raised to enable support of this User Story.