Be guided by what your tender nether regions tell you. Technically, if you slide sideways in the seat your are exceeding the safety limits of the creation. Seriously, I think a few of us have ploughed the same sod.
It was commented elsewhere that the Barnett Williams design seemed a bit complicated and that complication may have been related to the tube bending requirement. I commented elsewhere that I am about to restart my eLecTricks build and I plan to have a modified Barnett Williams seat. The mesh cloth won't fold around a bent(?) tube frame bent with a tube bender.
As it turns out, the trike body will need some framework around the seat area. I plan to spread the side stringers out to trike body width, using it as a reinforcing frame as well as the seat frame. Bending the frame will be a matter of kerfing the tube, bending it, and welding at the bend. The mesh will form a hammock into which I should sink quite comfortably. Lacing to the frame should ensure that lumber support, etc., is maintained.
Anyway, that is the plan but, we know what happens to plans.