There's a lock on the market that looks like an oversized pair of handcuffs... I know a couple of B owners that use it
Those over sized bondage Hand Cuffs are sooo easy to break,,, the link between the two half's is the fault. I watched demo's of how easy it is to break those things ( seconds ),, but used for other things those cuffs are really cool
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 Thread1Thread2Thread 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.