The fact that it's controlled by Hijack really sounds interesting but what exactly does your schematic do?
Does it turn the fan on at a preset temp or does it raise the speed as the empeg gets hotter?
If it only turns on the fan at a certain temp, say 50° or so, what's the hysteresis? Wouldn't want to have the fan turn on / off / on / off at full speed every 2 minutes. Think I'd rather have it go faster as the temp gets hotter.