Of late, my feeling, and that of a close colleague, is to install a diesel genset on the defender, and then mount an electric a/c unit, as many of you have suggested.
Diesel is required, as my Defender 110 is a diesel, so I already have the fuel with me. The genset could be mounted underneath or inside a fender, and vented easily.
The advantages of a genset are many: power backup, built-in inverter, a/c, heat, winch, battery backup, etc.
So the search shifts from an electric a/c...to a small diesel genset, plus an efficient electric a/c unit. Now, it looks like most of the smaller (Honda-style) gensets are...petrol!
Jeez. I can't catch a break.
Welcome your continued feedback.