I'm a little confused. Why do you think the headstock will snap under string tension?
I've seen any number of broken necks from being stepped on, run over by a bus, shipping accidents, etc., but I've never seen one break from having strings tuned to pitch. If your scarf is joined correctly it should hold with no problem, and there are thousands of guitars built without volutes every year that hold up just fine.
Maybe I'm just not getting it, but I don't see the problem.