No 1st hand knowledge, but in case you missed it, they do have a web page that says that there is primitive camping available and a reservation is required. On the web page there is a link to Google maps that brings up a satellite image of the area and you can see what the camp area looks like near the main entrance balloon if you zoom in enough.
Cypress Creek PreserveHappy Trails
Al
Al and Deedee
Spokane, WA
2008 Winnebago Sightseer 29R
F53 chassis w/ JMcKinley CHF and Brazel's UltraTrac rear trac bar
2006 Honda CR-V, Blue Ox Aventa LX, RoadMaster Even Brake