There are at least a dozen walk-in one-night-only sites just below the trail head. They're not usually full.
Is it proper etiquette to point out that people sleep in their cars, or on the ground - either by their car or "near" the walk-in campground - at the overnight parking lot?
I've got a Sienna, so it's easy for me to lay down the back seats and simply crash. Of course, I don't set up camp or take anything out, so it looks like my car is simply parked in the overnight area.
Last year, a guy pulled up next to my car on a touring motorcycle (BMW) around 8-9pm, threw down a tarp, pad & quilt, took off his leathers, and went to sleep in the dirt right in front of his bike.
The FS of course discourages this activity, but what they're really focused on is itinerant travelers setting up camp, etc. For all the traffic up in the WP area, as long as you're quiet and on the trail early am, an official would have to have a major bug to be hassling anyone.
Edit: I might be in the minority here, but I really like the Inyo county campgrounds (made possible by the DWP) such as Portagee Joe, Independence creek and Taboose. These campgrounds are great if you're traveling late and you don't need to acclimatize.
For example, if we're on our way to Yosemite, and we get out of LA in the afternoon, we almost always crash at PJ or IC - especially if it's after 8pm. Still hot, but real pleasant by the creeks, and no need to drive up/back into the mountains before once again hitting the road.