Hi Polgar,
Seems like you want a REST call with JSON for request and response, but behind the scenes you need XML document exchange with SOAP.
I would design my REST call with the correct input and output parameters and then transform the request into XML and the response from XML with server side scripting.
You will need to define your REST API with Swagger for it to be scriptable. We are working to make that easier.
Please keep us posted on you progress this is a good use case.