You can do that in "A" Class. My little Triumph is an "M" class, so handlebars and seat must be above the "top of the rear tire" at SCTA events. I'm not sure how they measure that at the front of the bike, but I think it best to keep them above the top of the front tire as well.
At BMST, the rule states above the top of the rims, so is a little more lenient.