You may be right. Based on further calcs, it looks like you'd need about 400 psi across the membrane at a minimum(based on the fact that seawater has an osmotic pressure of about 27 atm). I suppose you could use a lever of some sort, combined with the rolling motion of the water. It might be easier to use a centriuge system, although it would not be as passively operational as a shifting static load would be. A manually driven centrifuge would likely require more effort than it is worth.

My calcs could be wrong. Even though I am working on a big RO plant, I am not an engineer. I will have to ask the experts tomorrow and get back to y'all with this one.