The Wikipedia article on Torg describes a way that players could attempt to influence the metaplot of the default setting:
At the time of its release Torg's 'Infiniverse' campaign was an ambitious attempt at creating an interactive campaign setting. Subscribers to West End's Infiniverse magazine received response forms, through which they could inform WEG of the progress of their campaigns. Player input actually influenced the campaign setting through a 'rumor' system ('rumors' were introduced in Infiniverse magazines and published adventures, and the majority of responses would determine whether that rumor was 'true' or not).
I also found that there was a Deadlands Classic scenario called "Ghost Busters" that did this on a smaller scale: the back of the adventure had a response form that players could send in to indicate how the adventure turned out. Then after a certain date, the majority of votes would determine the true outcome and its impact on the metaplot. Specifically:
In that adventure, the manitou of a Harrowed Abe Lincoln took full control and caused him to do all sorts of mayhem. The response cards in the back of the adventure asked whether or not the gaming table killed Abe Lincoln or were able to save him. The end result was that the majority were able to save him and so the official metaplot is that he was saved in that encounter and is still kicking in the Weird West. Had the majority said otherwise, the metaplot would have been different.
I suspect that one major drawback to this system in the 90s was the fact that tabulating hundreds of response forms that were mailed in is a lot of work. But this sort of thing would be trivially easy to implement in the age of the internet (just go online and fill out a form, which would automatically tabluate the results). I'm curious to know if any company has tried this "write in to impact the metaplot" approach since the 90s (and if not, I'd be interested to hear why it's been avoided).