Here's the installation instructions for your tub spout. It shows you need a 3/4" female fitting 3/8" below finished wall. It looks like your spout also comes with a 1/2" adaptor or maybe the inside of the 3/4" adaptor has 1/2" threads also.
http://www.hayneedle.com/images/PDF/YOW3764.pdf
Since the spout adaptor fitting is threaded onto the wall fitting, maybe (and its a long shot), you could remove the o-ring, slide the adaptor over the pipe and solder the adaptor directly to the pipe from the inside of adaptor.
A 1/2" threaded brass nipple will slide over 1/2" copper pipe and could be soldered on.
There doesn't seem to be any water pressure being held back by the fitting for the spout since the spout doesn't have a diverter on it. I assume the diverter is part of the handle. Probably not the best idea but you might be able to epoxy the adaptor to the pipe if it can't be soldered.
Best thing would be to open up the tub wall and fix at the contractor's expense.
EDIT. Since the hex wrench size is 12mm which is a little less than 1/2" there might be enough material to drill it out to slide over your pipe with a bit of pipe protruding so it could be soldered from the outside. You would also probably have to cut some of the threads off to shorten the adaptor so it ends up in the correct distance from wall.