Shrader valve preload

Wont that affect damping, if i recall correctly, preload is only sag. On most forks (a fox 40 for example) to set the preload, you turn a dial (on top) that pushes on the spring (about 1 inch from nothing to full on) Adding air would mess with your spring weight as it compresses. There might be more to it, but an air shock just uses "compressed air" you would essentially be making it firmer if you were able to pull it off. see it might help your sag, but as you compress the fork for a bump, your 25 PSI turns into 200 and makes the fork ramp up, not a bad thing-if thats what you want, but i dont think it would be good for preload.

On a fork thats not designed to use air for dampening I dont think its a good idea. Like demo9 said, you will have all kinds of funky ramp up between the building air pressure and the coil that is trying to achieve the same thing.

the **** I've seen done to forks that contradict what theory and manuals says is not extensive, but its enough to say try it out and report back.

If your seals and stanchions are in good shape it'll hold air at least for a while.
Careful where you're drilling. I like a little bit of air preload to assist a spring.
Keeps it from bottoming without affecting the better-used range too much.