Sam,
The Dangerous goods legislation points to ICAO Doc 9284 for the definition of dangerous goods.
The only snag with that is that document is only available to commercial operators and not to Joe Public.
There are many things that the airlines prohibit, but are regularly carried on private flights like fire extinguishers and gas bottles in life-jackets and rafts.
I can't recall that the act has ever been invoked for a private flight. It would be hard to convict someone for not obeying instructions that he was not permitted to access.
The 'endangering aircraft' rule is far less specific and would be up to a court to decide what is reasonable. A life raft or extinguisher activating in a small cockpit is a significant hazard, but one that is readily accepted