Isn't part of the appeal of the 5/8" bar to have the big, "beefy" look? I think that a 1/2" bar would look pretty weak. It might be very strong, but it wouldn't have the same appearance of the 5/8" bar. Have you thought about using 5/8" tubing instead of solid bar stock? The tubing would probably be a little easier to bend, would still give the same appearance, and would probably be nearly as strong. (You'd have to do the engineering on the strength part) Plus, it would be a little lighter - that means lower material cost, lower shipping cost, easier to handle, less weight on the bike, etc. Just a thought.
Anyway, before you switch the diameter, ask yourself what your purpose is for making this bar? If it was to have a thick, heavy-duty appearance, then stick with the 5/8" material.