Yes, you can use the US site to secure seats after the award ticket is issued by AA. (Note that unlike US, with AAdvantage there is lag between the time you initially book and the time the ticket is actually issued. This can be minutes, hours or days.)

You'll need the US record locator to pull up the reservation and assign seats at usairways.com. In my recent experience, that appeared on the ticket once it was issued, so I didn't have to call in to get it -- which one frequently had to do with Dividend Miles-issued tickets.