Yes, you can camp at Whitney Portal at the backpacker camp area. I believe there are ten sites, first come, first serve. I have never heard of it being completely full, and since it is a backpacker area, people will usually welcome another person who needs a spot to throw their bed down.
And down in the Alabama Hills, many people use the area to camp. It's BLM land. But that is mostly in cooler months or winter, because it can get hot in summer.
However, once you are inside Inyo National Forest boundaries, but not in the walk-in campground, wilderness permits are required.
I am curious, Rogue, what you mean by "dispersal" camping. If it just means camping out of sight of anyone, and then removing all traces of your being there, I have done that along Tioga Road in Yosemite several times. (But I know that is not actually legal there.)
