Just a thought, but don't the auto CTD powertrains already have electric-heated oil? I mean, indirectly, but the oil-water heat exchanger would transfer some heat to the trans fluid when the block heater is plugged in, right? Maybe you don't need any additional heat. A pan heater would warm it up more though.
Trans fluid is thin (at least thinner than 90W) at cold temps so I don't know that you need to heat it.