81 KZ1100A Fork Preload Adjusters

If you're set on adjustable preload, maybe you could just modify your existing fork caps? Like, drilling and tapping threads for a bolt through them, a couple lock nuts on top, maybe some sort of plate inside the fork that the bolt presses on. 

I dunno though, I don't notice my front forks behaving that differently whether I have a passenger or not - just the rear shocks. I have a spacer in my forks that has the preload about where I like it and I don't worry about it. 

That’s a DIY thing if you have a lath and can cut a groove for an o ring inside the borehole for the adjuster. Forks need to hold air pressure even if the air ride is removed. After pulling a cap, not hopeful of finding an adjuster given the 31.8x1.0mm thread.

Still researching the spring rate. Here’s Progressive Suspension’s spring

Im buying Sonic fork springs for my Kz750 E, stock trim. I’m about 225-230, I ride one up almost exclusively. Sonic recommended .95kg/mm. I don’t know anything about your factors but I’d think a .54 is to soft. 

Race Tech has a spring rate calculator that is model and rider specific.  About half way down this page ("Calculate Spring Rates"):

racetech.com/ProductSearch/12/Kawasaki/GPz1100%20(KZ1100)/1981

Guessing at some numbers, I came up with 0.95kg/mm which is the same as the number Mikaw mentioned, and the Race Tech springs come in roughly the same range (0.8 to 1.0) as the springs offered by Sonic.  It could be that the Progressive table you posted is assuming that you would still be supplementing the spring force with the air pressure system.

Also, the dual-rate/rising-rate/progressive springs that Progressive sells seem to have fallen out of favor.  The current preference seems to be to use single-rate / linear springs.