I don't think cutting that "backbone" longeron will weaken the structure, because of the way the lower part of the fuselage is a box structure. I would be concerned with adding much weight down the tail, as you move further back from the C/G. The other thing is maintaining the shape of the lift-up portion and keeping it fastened down while in flight. It's definitely do-able, and would look really cool (I'm a big fan of mods like this), but to make it worthwhile, you'd need to put a litter in back, detail the whole thing, and that adds weight. You'd have to add a pretty good bit of up front weight to re-balance things, and even though this plane has a lot of wing, you'd rather keep it light, I'm sure.