Has anyone thought of using pivoted quadrants, similar to those on the radius arms, on the subframe instead of the normal 90 degree curved guides? I am sure that such a mod could improve the amount of braking for a given amount of force at the lever, and should be fully road legal. You would need to tell your insurance company of course, but there should be no complications with the MOT or Construction & Use regs, as long as the mod was done competently.
Or, perhaps enclosed Bowden cables, as on most modern cars, all the way from the compensator (which balances the force to left and right brakes) to a bracket on the radius arm. Usually the last few inches of cable is open, with a clevis connecting to the operating lever. Modern brake cables last for a very long time, being greased during production, and with a rubber bellows type gaiter at the wheel end, and maybe the lever end too.