Attaching the thong to the cable on the handle will certainly work and will keep the lines attached to the pilot even if the handle is released during flight, the idea is not too practical if attending a contest that follows the rules regarding the pull test. According to the rules, the thong must be pull tested during the pull test procedure. Applying the pull test to the thong if it is attached to one of the handle cables is not a practical thing to do.
Keith