Though it wouldn't give the amount of heat out in total, why not use something like a Goalzero Yeti 400 powerpack, and then plug in a 300-400 watt fan heater. Yes it's about £400 - but you get a good portable power pack, no installation required at all, no exhaust etc - and you would get about an hour of heat , which would be ok for the majority of journeys - then use the car heater on a low setting if needed.... or is that not enough heat output?
You could charge this thing at work, and at home - so you'd always have the power on tap. I wonder if 300W over a 30 minute period would provide enough heat in conjunction to the heated seats etc.
Do you need a 2kw heater running all the time, or is a lower powered heater for a longer period going to supply enough? I liked the idea of the diesel heater, but it takes a lot of space, and you have to cut out the rear carpet mat and engine cover, which would cost far more to reinstate than the cost of the power pack, and no messing with diesel, and you keep the storage space too. I guess if you used that in conjunction with preheat mode, then 300w may maintain the heat in the car??? Even if it cant maintain room temperature, any thing close would be better than nothing.
... Just a thought.
2012 I-MIEV Keiko Silver 21K
2010 Insight ES-T 64K
2009 RX400h 4WD - 86K
2001 Accord Type-V (F23 manual)
2009 Hornet CB600F