I strongly discourage any tie-downs that contact the hull. It's the source of stress cracks in the deck lip you see on many boats.
The best type of tie downs are the ones that go from the frame corners (usually the castings or sidebars) to the trailer. They must be angled inward and the fore and aft ones must oppose each other so that the boat won't move forward or aft.
Like this:
However, it sounds like your system will work. It really doesn't take much to hold a Hobie Cat on to a trailer - even at highway speeds. The thing you're most worried about are fore and aft movement caused by hard braking and aerodynamic drag. Make sure everything is snug and follow the 1-10-100 rule - stop and check your tie-downs at 1 mile, 10 miles and 100 miles down the road. If nothing has moved, then your system works.