Hey Neil, great work with the Xa. I'm envious.
I was wondering if you've made any mechanical and/or electrical modifications. I'm planning to replace my (rather worn out) battery with deep-cycle ones, possibly a pair of Optima yellow or blue top (only difference is the blue has threaded studs in addition to the standard automotive ones). I'd relocate them to the spare tire well to allow 2 to fit, as well as to improve front/rear weight balance. My spare has been replaced by a tire-plug kit, a small air compressor, and my trusty AAA card for really bad blowouts.
I've picked up a couple of solid-state relays and plan to cut power to the alternator's winding unless the throttle is at idle or I flip a dashboard-mounted "just in case" switch. Then I'd top off the batteries with a smart charger at home.
Theoretically this would allow the alternator to free-wheel unless I'm coasting, decelerating, or stopped, in which case it would use this otherwise wasted energy to charge the batteries. Do you know of any cases where someone has done something similar?