Well the massive jib on the 18 is STILL a blast downwind... you gotta get w/that jib flying a hull downwind... she'll move out!
otherwise, sure you can put a spin on the boat... I put a NACRA F17 spin on it and what a blast.
just pretty much moved the whole system onto the H18.
basically, just copy the F17 design, that's what I did... works great, tons of fun.
but I still also love the boat without the spin, under just the standard jib.
when it's wild out, I just use the jib since the boat wasn't designed for a spin...
but when it's light air, the spin opens the boat up nicely w/o overstressing it... best of both worlds.