My most recent experience ticketing an award on US didn't involve a stopover (although it did represent my greatest flown mileage in a single award), however my recent experiences have shown that they are getting far more by-the-book than they ever were before. Lots of referring to the *A website for valid routings and such.
As to the specific question of stopovers in airline hubs, I was quoted that two years ago and nearly missed a flight because of it (long story and even longer phone call to get it sorted). So they're serious about it. Your option is either to accept it and move on (no stopover), change carrier now or at the last minute to get the stopover you want, or hang up and keep calling until you get someone to ignore the rule. IMO, it's a very dumb rule, and certainly not one that's a requirement of the *A. Just don't get something noted in your PNR.
Another option, depending on the airports involved, is to simply do an open-jaw. If you want to post or PM the routing you're attempting to ticket, we might be of more assistance.
Good luck!
Sean