US Airways participates in the Star Alliance Upgrades. But, you have to be on a full Y or B coach fare. If it's a 757, and the miles aren't a big dent to the OP's UA account then it's worth trying to redeem. I don't see anything that says the *A upgrade awards are for TA or TP travel, although I suspect that's where most people try to use them.
http://www.united.com/web/en-us/cont...rade/saua.aspx
Strangely enough when you click on the link in that link that says "view the Star Alliance Upgrade Awards chart.", it just links you to a general award flight view.