Honestly partner reward availability fluctuates so much that a straight forward answer is not really possible... I have booked many times on partner airlines using CO Onepass miles, including Lufthansa, Asiana, Singapore, SAA, Thai, AC, but never specifically USAir. In the cases I have booked partner rewards, there is really no trend of when or even IF reward availability for certain flights would show up for the CO Agents or on the web for one pass miles. If you absolutely must travel on specific dates, best bet is to either pay for the exact flight you want, book on another airline if available with points, or take the gamble of waiting it out and checking availability often.

The Good news is that I have quite often booked partner rewards at CO Onepass Saver rates, but again... this is not always the case.