Wouldn't two round-trips, throwing away the extra segment as well as the return, come out cheaper? If you don't put in your frequent flyer number you should be ok, although the moral qualms (if you have them) don't change. Throwing away the return of a round-trip (although not with a hidden city) is something that was obligatory at an agency I used to work with. It was in the day of printed tickets, and they didn't even bother giving the traveler the return coupon. Of course, things may have changed ...