First off, the 333 has plenty range, and is currently flying the PHL-FCO route. That route is only 4400 miles, well within the 333's range, even with a sizable payload. It was however, a bit of a stretch for the 762s.
As for the routes to Japan, unfortunately, US did not have the equipment at the time to run the route, and had to cede to Alaska Airlines as the 7th airline to be allowed to fly to Japan. What's weird is that Alaska hasn't started service, and neither has TWA. I wouldn't doubt that it is possible for US to get that authority back.
If somehow, US Airways does get into the Japanese market, 333s won't cut it of course. However, the 332 would make it, and I think that's a better option than going to the 4 engined 340.