hi Johan.
taper rate wis best managed by attaching the "lam" to a pre-tapered sled- but i cant see haw Dean has set his rig up to control thickness. maybe he is just moving the beltsander closer to his fence and re clamping
ensuring that the belt is always perfectly vertical, and // to the fence might be a bit tricky - but i am sure that he has figured that out. as that would lead to a lam with uneven thickness- side to side.
Dean will respond soon enough.
i am sure if you got hold of Willem- using his router jig- he could build you any sled with whatever sort of taper rate you need.
the sleds i got from kenny are made from bamboo flooring- nice and stable, and dont warp and twist as easily as "real woods"
some guys have them made from aluminium- machine shop would have to do that for you.