Well I guess I should have posted sooner, because as usually happens once I post I find the answer.

I only stumbled upon this while using JSFiddle. I noticed that they had dependencies you can add, among them was the JavaScript port of libphonenumber (which I'd already tried to use). While it doesn't load correctly in JSFiddle because the connection is refused, the link still works to a minified version.

See here: Minified JavaScript of libphonenumber-js

Once I dropped this in place, and modified my code a bit, suddenly we're off to the races and everything is golden.

<script type="text/javascript" language="javascript" src="/static/js/libphonenumber-js.min.js"></script>
<script type="text/javascript">
     $(".phone-format").keyup(function () {
            var val_old = $(this).val();
            var newString = new libphonenumber.AsYouType('US').input(val_old);
            $(this).focus().val('').val(newString);
    });
</script>

I'm really still curious if I just missed something in all the searching I've done over the last couple days, because I don't feel it should have been this difficult or require this much reverse engineering to be able to find this version of the package.

Oh well, hope this helps someone!

I've got a custom build (currently 220KB) that I use for my International Telephone Input plugin, with plenty of helper functions exposed. Read the source for details.