If I understand the exercise you are describing, it works the abductors (outer leg). What you propose as a substitute would work the adductors (inner leg) if anything. To work the abductors, lie on your side and put a weight on the upper leg (I prefer to put the weights over the knee in this position, to reduce stress on the knee from having the weight at the ankle. You could also hold a dumbbell on the leg above the knee). Then lift the upper leg up.
You could also do the strap exercise by using your own resistance. Keep your hands outside and above the knees and as you open your legs, push in with your hands. The resistance you get will be limited, but may be enough for now, depending on how strong your abductors are.
For all who have received the new DVD's and do not have a tubing stap: I have been substiting my hand weights for most exercises and while doing the lower body section of Basic Step, I came across an exercise using the tubing strap for outer thighs which consists of lowering the thighs out to the side and bringing them back while lying on back while using resistence of the tubing stap. Any substitution ideas? I thought maybe I could hold my weights to the outside of my thighs while lowering my legs out to the side, but thought this might be akward. What do you think?