The reverse breakdown voltages are normally very large, such as 100-200V, well in excess of what a battery or generator can provide. Those voltages can be purposely lowered, as in Zener diodes or in photodiodes, but that is normally not the case in LEDs. They can be safely used in reverse polarization - they will just not light up.
I don't know about that "bulb" specifically but common Cree LEDs are rated at 5v reverse.
http://www.cree.com/products/pdf/XLampXP-E.pdf
Dynos can put out 100v.