Dammnn, you can't sleep on X-mas tree anymore?! Such is the plague of gentrification... When I was there a couple years ago, the legality of camping there was quite hazy - the cops generally had better things to do than going after kids camping on the island - out of sight, out of mind seemed to be the theory there, and it seemingly worked incredibly well.
As far as stashing packs...that's a damn good question; I was living in a van with a lady-friend while I was there, so that wasn't an issue. As such, I might suggest making friends with some of the 'van people' - there are a ton of people that hang out with their vans on Higgs Beach allllll day. Perhaps one of them could caretake your shit, if they're good peoples? 'Cause yeah, I'm not sure where you would stash your stuff... maybe in the vicinity of the marina on the other side of the island (sorry, the name escapes me right now) - that area had about the best selection of shrubbery and whatnot, and if you go further away from the marina (like towards Stock Island), it's even more off the beaten path...