Does the fork have "caps" at the top which can be removed, so you can access the inside of the stanchions? If they do, perhaps you could cut a piece of steel pipe, or a wooden dowel, approx. the same size as the coil springs, and put it over the springs, so when you put the caps back on, the springs will have more preload on them, making them stiffer. You might have to cut a few until you figure what length works best, but I think it would work. Another thought is maybe to put an elastomer in the stanchion, to introduce more preload--went looking, wasn't sure what I might find, but this looks interesting: http://faqload.com/faqs/bicycle-comp...ork-elastomers