I've not yet made it up to the spot I mentioned to jEFF, Pleasant Valley. It is, however, about the same distance as soda springs from civilisation. It's been a few years (six or seven) since I was last there, but at that time it was a very nice, almost unheard-of campground. That isn't to say hard-to-find, but you have to know that it's there. Another advantage is the campgrounds are off the road that leads UP into the Snowqualmie National Forest, where there are plenty of alternate camping sites of the "pull off and camp" type (i.e. bring a shovel).
I'll post more info when I'm next at a computer; at the moment, I'm in Portland typing this on a damn phone keypad. I'll also make a scouting run up there if anyone is interested. My plan is to be back in Western WA this evening (thus making it tough to see WTF is there), but I should be able to cruise that way tomorrow.
Should soda springs or any other Rainier-area campground be chosen, the Bradbury compound (my North American headquarters) is where I'll be departing from, and would serve as a decent rendezvous point, with easy parking for three or four vehicles if need be.
Oh, and I have acquired a [soon-to-be]Maui Cruiser, "Schmancy":
I'll be wanting to do some light 4-wheeling before I ship her to Maui next week (nothing like the last time we went off-road in one of my jalopies), and this seems like a good opportunity for exactly that.
Chime in on what you think of my idea, and I'll make a run to either spot to do a little recon tomorrow/Wednesday.