Well, you could use a helper spring. Our Baja uses something similar so that it can absorb big bumps but still have the ride rate that we want. It's basically a softer spring that runs with the harder spring on the same shaft. You need an adapter between the two springs in order for them to stay aligned. What happens when the springs compress is that the softer spring compresses the fastest. Once that spring bottoms out, the hard spring is the only one acting on the system and you're at a harder rate. This gives an extremely progressive rate for landing jumps.

The problem with this is that adding preload still wouldn't change your rate. You'd just end up with a higher ride height and a suspension that has less available rebound.

I actually like your idea about the secondary spring inside the main spring. You could have a shorter spring inside the main spring that would normally just be loose or just not against the main perches. Then, when you want to use the additional spring (for a stiffer suspension) you just crank the perches on the inside spring or somehow lock it into place. That way you could have the soft suspension around town and easier adjust your spring rate for racing.

Again, might be harder than it sounds, but the gears are turning.