OK, thanks everyone -- it seems like the consensus view is that it's perfectly legal to fly "around the world" on a roundtrip USA-Asian award. That makes sense because the computer offers itineraries in both directions -- and it generally works out to be the shortest flight times.
What was bizarre to me today was that -- even though I was already holding a ticketed reservation -- every agent I spoke to was CERTAIN that my itinerary was illegal, even though nobody could articulate why. And this was how I was treated as a 1K. Maybe it was just bad luck, but sheesh. Coming from the CO side of the business, I actually thought I would receive better customer service once the airlines was integrated (CO always seemed businesslike on the phone to me, while my dealings with UA's elite desk suggested they wanted to actually HELP me). I guess not right now. Maybe, as the supervisor told me, they're worried about being too lenient.