I don't know if a microballoons mix would hold a hinge. The classic way of doing it is to epoxy in a hardwood backer. To get back there, I'd tack CA it onto a dowel rod, check the fit, then put the epoxy on and let it set. You'd have to hold it there fora little while if you use 5 minute epoxy, then break off the dowel rod. It'll probably work best to put the epoxy onto the fuselage and just wipe a thin coat onto your backer piece so that you don't rub it off on the way in.