For now, I've settled on using HTML5 datalists in conjunction with a modified
control (I've submitted a pull request to the Github repo to merge those changes back into the master branch). All those changes do is persist the list
attribute to the inner
control so that it can utilize an HTML5 datalist.
Definitely not ideal, but, in the short-term, it'll function as a placeholder in place of a real typeahead control.
I also built a quick custom validator that iterates over the same Array collection as what I use in my datalist so that the
control knows if the current value is a selectable (valid) value or not.
Edit: The best place I've seen to bring up a first-party developed autocomplete is on the PolymerElements/paper-elements repo on GitHub. I'd discuss at https://github.com/PolymerElements/paper-elements/issues/26.