After reading the rules I believe adding a plate to the forward portion of the traveler to attached the main sheet block and swivel cleat is allowed - "5.4.1.9 Fixed-position or swivel base mainsheet cleat(s) may be attached to or in close proximity to the traveler."

However I also read the the following is prohibited. What is a mainsheet bridle system? - ".5.4.2.3 Relocating or changing the length of the mainsheet traveler or the use of an alternative mainsheet bridle system."