If anyone makes something it'd be cool tae see a diary of progress, recharging something like that on site would be a cool challenge too, we have plenty of wind and running water..
My gut feeling is just have spare batteries. I mean rugged easily changeable rechargeable batteries are something that have been sorted by the power tool industry for some time now.
Zoa PL1 is more the way forward than old style portable rope tows IMHO. Improvements in battery tech will only make it better as time goes on. It's one of those things where improvements in technology mean you need to stop and rethink what you are doing. Hauling a internal combustion rope tow somewhere is a lot harder than a 4kg device and a length of para cord.