Many "off the shelf" cafe style seats do have a hump, but very few align with the stock frame "hump". Further, the seat hoop has a slight concave radius to it and thus most fiberglass units don't sit flush to the stock hoop.
While there is truth to the tire clearance issue when replacing the stock hump with a flat pan, it is also most unlikely that the tire will contact the pan becuase cafe bikes seldom have a passenger compressing the rear shocks sufficiently to produce the conflict.
Lastly, to install a fiberglass seat, you'll have to remove the stock seat hinge plate and lock plate anyway, thereby rendering your frame "hacked and useless" in some eyes. Many install a new closed hoop because they prefer the look of it over the swaged butt ends from the factory. Better rear shocks and no passenger obviates the stock frame "hump". But a cross brace is advisable for suspension benefits.
