I just spent the weekend walking in this area and can give a bit of an update, probably too late for those in the thread, but maybe useful for posterity with the internet being what it is.
St Helena's Crater has been pretty much totally reclaimed by weeds - no open ground anywhere and no sign of recent vehicle access. I assume NPWS has given up the spraying program, and a wet hot summer has seen it grow wild.
Lost World Lookout has quite a bit of room, we had 11 people there, three tents and two tarps, and there was room for a couple more. You can camp at Bunyan too.
There's one nice big camping area next to Glenbrook Creek, near the western track junction to Martins Lookout, lots of room and water (which needs treatment).
Here's the crater.