It really depends on the alternator. If it is internally regulated, it will probably survive. If it is externally regulated, you may end up with an issue. If I was betting money, I would bet the diodes are OK. Semiconductors fail exclusively because of heat. Either the heat is generated quickly and they go bang right away, or they or they fatigue and fail over time. In the case of no load and no regulation, the voltage will rise till the diode conducts backwards (PIV). If there is enough current, the diode will fail. One thing to remember, the diode capability will likely match the coil capability. So as the current in the diode goes up, the ability of the alternator to put out more current diminishes. Coils usually survive overheating better than semiconductors, but the voltage will still sag - hopefully enough to prevent the diodes from failing.
ON EDIT: There is another senario. When you break the load connection on a coil, the voltage spikes. (This is the way your spark coil works) If you get multiple breaking and making of the load connection, the successive spikes could take out the diodes.