The question raised was about racing the SX in a class and not as an open class boat...Perhaps a new class, H18 Formula, is the way to go. Set a box rule that the H18 and the SX can conform to. That way you can get more boats on the same start.
Sounds like you weren't racing the 18 back in the late '80s, early '90s when new 18 platforms seemed to pop up every few years. First it was Magnum wings, then SX's, Formulas and spinnakers - not to mention the Hobie 21, 17 Sport, & Hobie 20, all of which, for better or worse, had a hand in weakening the 18 fleet. I think it has been pretty well proven that creating sub-classes only serves to weaken the overall fleet.
In my division (which I would guess has one of, if not the strongest 18 fleets right now) we will pretty much let any style of 18 come out and race whether you have a comptip or not, magnum wings, or even an SX. Unless you start to win, no one cares and guess what...the guys on stock 18s always win anyway...95% sailor, 5% boat.
By the way, I think the square top sails made by Whirlwind definitely look very modern & cool, and if I didn't race one design, I'd probably consider getting one. But allowing taller masts and open sailplans would kill the fleet instantly in my area.