I think that as a Missouri/Illinois resident you are exempt from any stickerage requirements that residents of KS would be subject to. In the unlikely event that you would asked by a ranger about a sticker, you could just show them your MO or IL drivers license and it would be a non-issue. At Clinton there is a day use fee for the park which like Larry said is in the 5-7 $ range. Where folks launch at Clinton is:
At Pomona, a windsurfing only venue, the launch is:
http://goo.gl/maps/m2H6Z -windsurfiers rent
this area out most weekends, and often camp right on site Fri thru Sun (you can see them on the sat view). There will be folks there this weekend for sure even tho it wont be the small sail conditions that Larry likes. They will be very stoked to see a windsurfer from IL if you decide to go. I wish I go myself, but can't do it this weekend...
I have stayed at the Motel 6 in Lawrence when windsurfing Clinton, but it is kinda sketchy, esp if you arrive late at night. My favorite place to stay is the extended stay deluxe in
Metcalfe:
http://goo.gl/maps/uPZpV -usually will drive at night, stay here, wake in the morning, check weather, then pick a lake to sail and drive out the last bit.
Most recently now though I have outfit myself for light car camping and just camp out at Pomona..
Hope this helps, have fun!
Hans