I know some people who have tried to do this in both Google and Bing with mixed results.
They did it by hiring people to perform searches. The theory being that the more a keyword gets used, the more likely it is to show up in the autocomplete.
The challenge is that usually people are trying to knock off a negative result. Something like "product name scam". In those cases, you are really fighting an uphill battle. When people see "scam" or something negative like that when they are searching for information, they are frequently going to check it out. So those negative autocomplete terms become almost impossible to knock off because of the amount of searches they are getting.
Hope I am making sense there.