Locking skewers might help. Maybe just the right U-lock?
You could use locking skewers on the wheels and then U-lock and/or chain & lock the wheels. Bike Forums has threads on locking skewers. Search "pitlock" or "onguard" or "locking skewer" and find threads such as Thread1 Thread2 Thread 3
Peter White Cycles sells the Pitlock
skewers which the threads indicate is the nicer (stainless so will not rust) but unfortunately more expensive.
I wonder if a U-lock of just the right inner throat width and depth could be locked around the main boom and not slide off. One way might be putting it diagonally with one part of the U on one side of the down tube (e.g. ahead) above the boom and the other side of the U on the other side of the down tube (e.g. behind) below the boom.