Ever since Chrome Beta users have been able to change the search engine that powers the omnibox (to include autocomplete suggestions, not just search results).
Changing the search engine that powers the omnibox is different than changing the behaviour of the omnibox, which is kind of a lot when it comes to predictive search, predictive completion and a bucket of other things that I wish you could turn off (or can be turned off for a while before Google decides you shouldn't ever need or want to turn it off). It's a target with a huge amount of churn. I agree that Firefox is a more sane target as a developer.