That's a really good one Sadie and in fact it's very similar to my favorite exercise for this muscle group. Imagine the same leg action but with the person in standing on one leg with a resistance band on the non-weight bearing foot, leaning forward with pressure on the hands (simulated cycling position). It's a tough exercise to do well while maintaining a neutral spine. So what I often do is get my clients to reach back with one hand and pinch a fold of skin in the small of the back while the low back is in a neutral posture. The skin fold should run horizontally. As the hip flexion motion is performed, if lumbar movement occurs it will cause the pinched skin to escape your fingers. This gives very good direct feedback about lumbar posture and stabilization during the exercise.

ETA - I really should do up some youtube vids of my favorite exercises.