Hello strake99...
Have you ever done any instrumentation work to determine the pressure difference between the cabin and the engine compartment where the inlet to the muff is (on your particular airplane)? I know you abandoned the effort, but I have heard of a few people that have had some success with the muff system, and I wonder why it works on some planes and not others. I believe that any leaks around the elevator will raise cabin pressure, possibly above that of the engine comparment.
To do the instrumentation work, you can either employ a water manometer, or an airspeed indicator. Run a length of tube from the area in the engine compartment where the heat muff is to wherever your instrument is located, and if using the ASI, connect it the pitot input, and leave the static input open (vents to the cabin). Go fly at various airspeeds, and record the readings from the ASI. If the heat muff area is actually at higher pressure, then you'll get good readings. Otherwise, you'll have to connect the pressure line to the static side of the ASI, and then you'll get readings of how much higher the cabin pressure is than the heat muff (and that will positively tell you why the muff doesn't work).
The manometer is nice because it reads plus or minus (+/-) pressure, whereas the ASI only reads plus pressure. The manometer is cheap, but you may spill water in the cabin...

Heat muffs work well on front engine airplanes because run is short, and the muffler and SCAT are big, but also because cabin pressure is low compared to ambient, and engine compartment pressures.. That gets the air to flow the right direction.
Regards,
Matt-