The IDEA has been discussed a number of times. But the only time I've read about it being used was on a flying wing plank style design. And there were special performance reasons for that related to soaring rather than aerobatics.

Don't think of it as tail weight being shifted back. Likely as not you will find that you need a big lump of lead in the nose. Moving nose weight back to under the wing works equally well as moving a mid location weight to the tail at shifting the CG. And it has the advantage that you're not adding anything that isn't already there. It's better to shift what you have to have on board than to add more to the wing loading.

I know the fuel tank is sitting in the way but nothing says that it has to be a single ball or cylinder like lump. If a couple of flat plate pieces slide along BESIDE the tank in tracks or pocket or on carbon rods if there's room then it'll work just as well as a single lump like lump on a rod. So keep the thinking free form and work at locating it forward.

I'd suggest keeping it high though. Especially if it's a mid wing style design. If it's a low wing then you can mount the sliding weight under the gas tank. The key here being that on a competition model were you're judged on the flying you will find that the vertical location of the balance is still fairly important. Not AS important as the longitudinal balance but it still affects how the model flies vertical lines and does knife edge. So keep all that in mind.

While the concept isn't new at all I do believe that you'll be one of the first, if not THE first, to use it on a big jumbo aerobatic model.