OEM block heater is a 400 watt inductive load if I'm not mistaken.
If you're up to making something work for you!
How about a propane fired camping water heater plumbed into the cabin heater circuit and having the auxilliary pump running to circulate coolant through the unit while it is running! You might have to do some engineering to the heater coil's input and output for connections but it should be doable. You would likely need to find or build a short adapter hose/pipe to reposition the standard disposable propane tank but again that should be a relatively easy fix. Get a 20 pound take and a refilling adapter for the small tanks and you should be good to go! Keep a tank hooked up under the hood, a spare filled in the trunk and you should be good for a couple days maybe depending on fuel consumption rates, when one tank empties swap it out and refill the empty when your home! Propane is a $1-2 a pound so that makes it managable, financially.
I assume the heater operates with a mechanical piezo electronic sparker/valve arraingment but even that might be modifiable to work on a timer so you wouldn't need to go out to the car and start it an hour before you need to start and drive. Maybe a solenoid could be the actuator of the initiator?
This one is the least expensive I could find with a quick search and its at Walmart!!!! From the description the heater housing itself is about 8" square and 10" high
http://www.walmart.com/catalog/product.do?product_id=4698021&sourceid=1500000000000003260410&ci_src=14110944&ci_sku=4698021
This larger more expensive one might even work better as it has a feature that turns it on automatically once the water flow begins! A seperate auxilliary coolant pump on an automatic timer would cause it to start!? A 3-5 pound tank in the passenger seat and your warm and toasty!
http://www.walmart.com/catalog/product.do?product_id=10363535&findingMethod=rr