Make the reservation using your US FF#, get assigned seating and so on. At the gate, walk up to the podium and say, "I would like to have this flight credited to my UA account, not to US. Is that okay?" Hand over your boarding pass with your UA card.

As for priority boarding, that might be a little trickier. Try whipping out your US Gold card and showing that as you board. <shrug>

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"There are those who travel, and those who travel well."