First there would be tickets (plural) for adding features to Twisted's
Request and FakeRequest from Nevow. Then there would be tickets (or
maybe just a ticket) for deleting the corresponding (then redundant)
code from Nevow.
My two cents here would be to move in the direction of making Request itself a data structure which could be manipulated in memory, rather than having it tightly bound to the HTTP connection, and then having a discrete test double.