Chris,
    What would be the possibility of getting/salvaging a bypass valve off a mazda cooler? something that sends cold thick oil though a bypass to the exit, and not through the cooler. As it heats up it opens slowly letting thinner, hotter oil down through the cooler, and once its HOT oil, it ALL goes through the cabin heater. This does add complexity to an already problematic and may not be the cure, but it gives you additional oil cooling capacity which as we both know is a larger percentage of the heat shedding equation that in non-rotary engines..

   If this isn't an option, then you could plumb (like you are asking) the front cooler to use hot coolant for cabin heat, and in my best estimation will work just fine for this function. The big outlet on the rear iron is the traditional heater outlet - its the hottest coolant... the return on a stock engine was on the water pump housing. The front oil cooler is more than robust enough to manage the thinner, lower pressure aqueous coolant.

Dave