Actually, jQuery tokeninput isn't appropriate. But these solutions exist...

http://yuku-t.com/jquery-textcomplete/
http://wick.sourceforge.net/wick_sample/
https://github.com/tactivos/jquery-sew
http://zachsnow.com/#!/blog/2012/jquery-mentions-input/

I am considering which is best to integrate.

We will integrate sew, which seems the most robust / well-maintained / prettiest.

This will mean adding a jQuery dependency into Composr. I'd hoped we'd not need to do this, because ultimately jQuery is going to be (is being) obsoleted by new web standards. But the reality today is most libraries are built as jQuery plugins and we cannot really avoid that. We will just have to keep feeling our way through Javascript libraries, changing/rebasing dependencies as the browser landscape changes.

So we need jQuery, plus these files:
https://raw.github.com/tactivos/jquery-sew/master/demo/demo.css
https://raw.github.com/beviz/jquery-caret-position-getter/master/jquery.caretposition.js
https://raw.github.com/tactivos/jquery-sew/master/jquery.sew.js

And I need to be wary of:
https://github.com/tactivos/jquery-sew/issues/6

I had to pick the one without AJAX support, lol. Fortunately there was a fork of it that did.

It is now working, using the same data source as the Composr username field auto-complete feature.

It is tested on iPhone.

Now I just need to test on IE8, as there was a report for jquery-sew (that I don't believe) about a particular bug on there.

I also had to make it work in CKEditor, which was really hard because it works very differently from a textarea - but is now done.