Getting request body as a JsValue in action composition

I posted in Stackoverflow originally here:

I have endpoints that handle querying of data. I want to be able to add a query object to a request based on the request.body. However, I need to access it as a JsValue but action builders only seem to support generic types so that means the request.body has type of A. Is there any way to do this kind of request interception and modifying?