the nut does indeed fit behind the nosepiece, but it is very difficult to turn. Crescent wrenches (even cone wrenches) cannot reach it. So I'll need to use a wider nut or some kind of spacing washer between the nut and the nosepiece.


I have also been planning (as ivan_yulaev also suggests) to put a round-headed machine screw (drilled for screwdriver) in place of the bolt. This could screw all the way into the back of the plate that connects to the rails, and then turning the nut to push the nose of the saddle away from the rails (and thus add tension to the leather) wouldn't require "holding" the bolt with a screwdriver.
Also, the replacement bolt should be a bit longer, as the stock bolt one would run out of real estate pretty quickly if the leather proved stretchy.