Ok, for anyone trying to do this, I solved my own problem. RadAutoCompleteBox has a FilterBehavior member which is based on the IFilteringBehavior interface. Simply create a class which implements that interface, like this:
CustomRadAutoCompleteBehavior : IFilteringBehavior
// custom setup code
searchText, System.Collections.IList items, IEnumerable<
textSearchPath, TextSearchMode textSearchMode)
// custom filtering code returning IEnumerable of objects to display
and set your RadAutoCompleteBox's FilterBehavior to an instance of the new class, like this:
There you go, now you can filter the autocomplete items however you want, not just on a single string member.