I understand that the lookout is available for overnight use on a first-come, first-served basis, but: is there a way to know at the trailhead (i.e. a sign-in form) whether or not there is space available?
There is no registration at the TH... or any clue of anyone's intentions.
Josh Lewis and I spent a night there. Two other male hikers got there before us. They were quite willing to share the lookout with us. There's a full sized bed to throw your sleeping bags on. There are also a couple of thick mats under that bed that can be moved out to sleep on.
If you don't want to share - take your own tent.
I don't know the squatter's rules... but I assume there's room to sleep 6 people on bed and floor in a pinch.
Lookouts sound cool for spending the night, and they are a little oasis in winter, but if the weather is nice I much prefer camping now. Lookouts tend to be infested with mice. Plus unless you arrive late in the day and take off early, people will be filtering in and out while you are there... so not much privacy to be had if you want that.