Hi
I don't see anything for auto complete to restrict within collections as Search and Promote only storing the list words while doing index based on the fields we asked to index.
And in the response it is returning only the words and I think filtering is not possible
acObj.generateRequest = function(q) {return "?query=" + q + "&max_results=" + acObj.maxResultsDisplayed + "&beginning=1";};
You can check below link to see which has details of customizing auto-complete results.
https://yui.github.io/yui2/docs/yui_2.9.0_full/autocomplete/
Hope this helps !