For the purposes of upgrades I think you have to regard USAirways as any other Star Alliance carrier if you have non-US Elite status...

It's not spelled out on the website because it does't really exist as a published benefit, as I understand it.

It's pretty much not happening for upgrades: the best you can do is 'request' preferred seating by phone with US (50% success rate for me: it's also not a defined benefit, AFAIK) or choosing preferred seats at T-24 when they become freely available at OLCI

I think they offer buy-ups for 'tens of dollars' usually less than the equivalent UA buy-ups.

US routes tend to have a lot of US elites so UDU has about a zero chance of happening if you're UA Premier (Star Silver or Gold) unless you use a Star Alliance upgrade from Y or B with miles - but many would consider that a waste of money and miles!