If it's really a 6 volt horn you could easily overload the wiring on the bike thereby putting Mr Lucas's theory to the ultimate test.
In case you didn't know, Mr Lucas theorized that all electrical current is conducted thru the wiring by smoke.
The smoke was contained by the insulation wrapped around the wiring and if this insulation was damaged the smoke would instantly leak out resulting in a failure of the part (in this case, the horn) to operate.
I'm not sure why a 6 volt horn attached to a 12 volt system will damage the insulation but I suspect it will try to force more smoke thru the insulation than it can possibly carry. The extra smoke will therefore cause a rupture in the existing insulation resulting in massive clouds of smoke escaping.