Actually, there are several 600yd KD ranges in the western Washington region... have you tried looking on the state rifle & pistol association site (www.wsrpa.net)?
Ones that come to mind...
Paul Bunyan (Puyallup)
Upper Nisqually Sportsmens Club (Eatonville)
Seattle Rifle & Pistol Assoc. (Machias)
Tacoma Rifle & Revolver (Tacoma)
Then when you get on the 'dry' side things thin out in a hurry...
North Central Washington Gun Club (East Wenatchee)
Spokane Rifle Club (Spokane)
If you have Google Earth installed you can go here:
Google Earth Community: Fullbore/Highpower Facilities
and download a data file that should open in Google Earth... and show you the location of most of the major KD ranges in the US and abroad.
As far as 'field' shooting... short of finding a section of public land w/ a powerline cut thru it... I think you're pretty well SOL. I was under the distinct impression that the local law enforcement had clamped down hard on exactly that kind of shooting (people going out in the woods and banging away) due to the population density and vandalism/littering. Still fairly accessible over on this side of the mountains (even so, there are a few 'holes' I wish they'd shut down).