when it comes to cardio equipment, there is none that does not include the movement of hip flexion. When I see people on the elliptical trainers, I often see poor posture with forward flexion at the hip. A person with tight hip flexors would more likely to assume this posture. On the other hand, they would also do that on the others, such as treadmills and bicycles.
From my own experience, people like elliptical trainers the best of all the cardio equipment, and you will thus see more compliance with your exercise recommendations. I would instruct the client on a good stretching / lengthening program and stress the importance of doing it after getting off the equipment. At the same time, you can work on strengthening the muscles of the back in hopes of affecting better posture.