I've had several different situations.

When booking this winter's vacation, I initially booked an O award for my wife, with one segment only available in X at the time of booking. We then waitlisted for O for that segment. (We eventually waitlisted for I for all segments, and when the last one cleared, UA repriced the itin, refunded the mileage difference, and charged a slight bit more in taxes as they had changed between the original booking and then.)

For another one-way award, I booked I for 2 segments and X for one, and waitlisted that one for I. These were all UA metal, which helps, but we did the same on a mixed UA/LH/SN itin as well without issue (obviously we waitlisted only the UA metal segments, as you can't waitlist partner flights--you just have to keep checking).