I don't see why not - dacron is polyester. when in doubt just make it thick. even if it's braided it should be fine
Adam Karpowicz mentions using copper wire as a tillering string... anything stiff enough should work. Any natural fiber, or any synthetic fiber other than nylon that's no stretchier than polyester I guess
If your bow is heavy, it will feel very stiff. Even if it's straight, 70lbs feels very stiff for me to string. You probably don't want it to be too easy to bend at floor tiller or you'd end up underweight after tillering. Just don't break it or something.