When you dispatch using a wrapped request/response object, pluto doesn't preserve the wrapping when it executes the dispatch. I.e. it upwraps the request/response and dispatches on that. This prevents portlets from filtering request/responses to/from dispatched/servlet entities.
It would be nice if we added a TCK test for this case as well. The spec is clear that one can use a wrapped request/response to dispatch to. Though it doesn't specifically state that this must be preserved, it not only is the reasonable interpreation/expectation but is what clients will be counting on. Hence for the sake of interoperability, having a TCK test will catch this problem early.