Is there a random event mod for an asteroid/Comet strike at all?
Not just a fragmentary one, but one based on the realistic odds of getting slammed.
Currently, the NEO sites tell me that every 1,000 years there should be a tunguska sized event* with the biggies every 65,000 years or so
(we are overdue now in the real world as well)

* Assuming Tunguska was an impact event, of course. It may not have been.....
This would give as many as 6 in a typical game.
Using the real odds would give a rare - but realistic - bonus.

Well one problem I see if that before 1800s AD you wouldn't know what an asteroid was and that they can impact the earth so any event like that falls under "random act of god" type random event. Also another thing to consider is that given the fact the earths surface is soo much water, most impacts will be seabased and the resulting wave from it can be just another earthquake in the grand scheme of history (there is a theory that the great flood recorded in so many diffirent regions and cultures folklore is really the result of a meteor impact in the Indian Ocean).