I have had coils made with a taller free height, to stay captive at full droop, effectively a variable rate coil, but the first rate is only 90-120 lb rate, so its compressed at ride height, and keeps the coil trapped at droop, while offering some resistance, when the vehicle comes back onto the coil, in a wheeling situation.
We have managed this by tapering the wire, for the last coil section, to a smaller dia.
I have had these coils made in front and rear for 80/105 for heavy touring wagon, like 864 eq rear, but taller free height, and slightl;y softer rate, so car sits where we want and wont shake your fillings out when some gear is taken out, and we have done an equivelent front coil as well. I also have them for patrol front and rear, in a heavy tourer, and the process of the current coils, with softer rate, more like 860 rear rate, is to spec the spring, and get some of these made as well.
in this pic, you can sort of see the type of design, from my old patrol front coils, which was before we went tapered wire, so we could reduce the amount of upper coils.
This pic below, was when i started playing with the idea back in the mid to late 90's, with my then almost new 80, after having Koni bus shocks re valved to suit the car, in the lengths I wanted, at the time..
Our valving, for the silver series we have put together, is aprox 15% stiffer, for the more lift, and heavier vehicles, over what we had been using in the white series, and probably 20-35% more than some OME shocks, but also works differently with viton seals,a nd better RFA on the shafts to reduce friction, as well as shot peened internals, and a different oil. Build quality of these shocks would be in the same range as Koni, but I prefer the way these shocks work, with less of a pointed "sweet spot" and a longer flatter curve where they are at thier best, especially running larger heavier rim/tyre combination.