Couldn't take the heat in that water closet!!
Replaced the factory vent with an adjustable dampered diffuser from
http://www.dwincorp.com/index.php
It requires a 4" X 2" reducer, also available at the same site. (this stuff is all relatively cheap). Some creativity required, as the vent is actually made for a 4" pipe. The reducer has to be installed lower than normal with screws to allow enough room for the damper to operate. The 2" pipe gets installed with a clamp, then all wrapped in alluminum tape. The whole thing is accessed from the basement (at least our model).
Of course, we replaced the remaining floor vents with adustible damper types from Home Depot.