For a situation with no AC power and no generator I think I would go with the Red Dragon 12VDC powered preheater.  It uses propane to generate heat, a lot of heat, their website says 49,000 BTUs/hour.  It also says the 12VDC blower pulls 4A so I'd bring along a separate battery to power it.  They say it will preheat an engine in about 15 min at 0 degrees F.  They don't say to what temperature, whether that's the cylinders or the oil, and what size engine.  You'd need to do a little experimenting to find out how long your plane needs to be heated.

Aircraft Spruce has better prices than going to Red Dragon's website (http://www.flameengineering.com/Preheater_12voltDC.html)

Tom Gourley