My last XJ (2000 Limited, 4.0, AW4, NP242) ended up having the factory high idle switch installed; this was an option on police-package models. Doing so brought RPMs at idle up from around 800 to 1100, and was useful in two situations:
- Extended running of electronic (mostly radio) equipment
- Combatting heat soak while stopped and idling in desert (120°F) temperatures
Obviously this isn't a direct parallel to the KJ, but there are some similarities in terms of theory of operation. Taking your points in relation to those:
If your main concern is coolant temperature (and thus cabin heat) in Winter, I'd suggest checking your cooling system first, and definitely consider upgrading your thermostat; the HDS Model 001 is a good choice as you can run a thermostat that's significantly warmer than the factory 170°F unit. In the -30°C temperatures you're talking about, doing so may be necessary to keep the coolant at a temperature that can retain enough heat to keep both the engine and cabin warm.
Also, have you tried blocking the grille? I didn't see mention of that in your earlier post, but, if you haven't, may be a good first step to investigate.
As wwdiesel mentioned earlier, the Webasto heater might be worth looking into. You're going to lose a lot of coolant heat while parked, and finding somewhere to plug in the block heater isn't always a possibility so being able to heat it without need for a power outlet would be a huge plus. I also question the effectiveness of the factory block heater in -30°C weather - it'll probably work, but take forever to generate a useful amount of heat.
As regards electrical loads: high idle can help with that, but it will obviously do so at the expense of higher fuel consumption. Your call on whether or not that's something that should factor into your considerations, but given the coolant issues those should probably be given priority.