I was thinking of printing a heat transfer; the width of the sleeve, x 2 would give you the length that wraps, maybe shave a quarter inch off of that. Then, you could fold it in half, insert the sleeve up to the crease in the transfer, and press it; it should do both sides at once, and no lining up 2 prints, not that it's any less labor. You might consider the different lengths for each size shirt, or make them all large enough for the largest shirt in the order, then trim them down for the smaller sizes. I have done some line up prints going over the shoulder before, and laser pointers helped a lot with that... good luck.
Steve