Of course, after I post I find something useful. The important part of this is:

The action performed by the POST method might not result in a resource that can be identified by a URI. In this case, either 200 (OK) or 204 (No Content) is the appropriate response status, depending on whether or not the response includes an entity that describes the result.

So I suppose the problem is that returning a 'text/plain' stream as the result of a POST is not something that can be identified by a URI. In other words: POST should only return HTML, then?