Pointers for better integration of Spring Webflow and REST exposure

Question

As mentioned in another answer, our use case for Webflow is a bit unusual. Instead of a traditional MVC approach driving UI, with a little effort we exposed SWF as a RESTful HATEOAS API (with some caveats). Our Views are Collection+JSON format with links modelling SWF transitions.

The problem is that SWF tightly binds to the MVC pattern of returning ModelandView, with the View states resolving a view and calling render(). As such till now our choices have largely been to expose the API via a JSP (yuck). We're looking to use Spring Boot (no JSP) and moreover use RestController and Spring Hateoas where instead we want to return ResponseBody<> or ResponseEntity<T> from a controller method, with that being an object formed from the View(state) - i.e. get the Model but no view and generate C-J or HAL+Forms or similar from it with transitions as Links.

We use XML flow definitions (essential to the solution) so I figure either I'd have to

1. Define a new Flow xsd with custom attributes and monkey with the ModelBuilder code and View implementation. Seems a lot of hackery.
2. Somehow define a do-nothing MVC view and defer to the customer controller for the HttpResponse when entering View, but how?
3. Write a complete custom FlowHandlerAdapter (this might help alter the POST-redirect-GET behaviour but won't tackle the ModelandView bit)
4. "Insert some more elegant answer here"

What is the best route to pursue?

Before this whole thing is considered lunacy, think what SWF brings to proper REST API exposure of business logic - not just another tedious boilerplate example of a CRUD repository API, but loosely coupled declarative workflow. I looked at Spring State Machine also but frankly it seems miles behind SWF on key aspects and doesn't help with state exposure as REST.