Threads like this one are why I love this forum.
It seems to me that one could put a fixed weight on the end of the lever arm and simply move the fulcrum around to get the force desired. I.e., avoid the inconvenience of changing weights at the end of the arm. Perhaps a bit more sophistication would be involved around the movable fulcrum.
BTW Ozzie, we have VERY similar workbenches. I've got a large blue Record vise in the same position, maple inserts in it, and my bench looks similar too. Mine's made from a section of a maple bowling alley, with 4" locust legs.