Did it last year..
I favor SKS fenders and there is an adapter you can buy to mount them on bikes equipped with Headshocks. The part costs $6 at A.E. Bike and will allow you to use the Shockboard (older) or Shockblade (newer) fenders. Here's the link.
http://aebike.com/site/page.cfm?PageID=30&SKU=FE1046
Note: I had a '98 F500 with a P-Bone Headshock, which I believe was an early model, if not the original. As a result, it didn't have much clearance (maybe 1 to 1 1/2 inches) and the Headshock adapter did not work for me by the time you mounted it and the fender. It also wouldn't fit with my V-brakes. My solution was to cut the fender in half (there is an upper-front portion and a lower-back portion) and, using some misc. hardware/brackets I keep around, I was able to build a little mounting bracket that fit on the bolt hole in the front of the fork (must have been there for accessories or something). I rode that solution all through last winter without a problem. I still have the Headshock adapter, which I can't use because the bike was stolen. If you want it, you can have it.. private e-mail me your address. You could mount it and get a Shockblade from a local retailer and try it - if you can't get it to work, take the fender back and it won't cost you anything.
I don't know anything about the air valve but assume that's got something to do with air for the shock being pumped from the bottom side? I don't know how often you pump your shock but, if it's not very often, you could go with the solution I just gave you and just have to cut/re-zip tie the adapter every time you need access to the air valve.