A hybrid system like this is certainly possible, but there would be a lot of safety concerns so you need to be sure you know what you are doing. A full charger in a 6" enclosure would not be necessary, you could build a smaller system directly into the existing enclosures. In fact, DC power at 48-72 V would be plenty on its own, depending the amount of copper you have in your tether and the charge current you are looking to achieve, and would be much safer. It would be possibly to be constantly topping off the battery without interfering with anything. We are actually working on a hybrid power system like this, there is no estimated date of availability however.
Here are few topics that discuss both hybrid and full topside power over tether in more detail:
I have also sent you a private message regarding the other concern you had.