I do not yet have my FEH (its on order) But as I have read thru tips and blogs I understand that operation of the climate controll on "Defrost" requires operation of the AC compressor thus it also requires operation of the ICE - and will it prevent the ICE from shutting off and going EV. I have one word for that: RAINX I live in Colorado and use this stuff all the time. There are two products ANTI-FOG for the inside of the windscreen and also normal RAINX for the outside .... Snow does not stick and brushes off, Water runs off faster than the wipers can even get there, and the Anti fog on the inside prevents vapor accumulation "fog" on the windshield...... with these products I seldom use defrost mode as I try to hypermile my existing (non-hybrid) vehicle as it also runs the compressor in defrost mode.
Pretty low tech... But it works!!

I would like to find a way to be able to run FEH air vents in the defrost mode without the compressor as even a simple fresh air flow can very often address interior moisture on the windshield - but i bet its a lot more complex that putting a relay & switch on the compressor clutch.... the engine management system must be very integrated.