Hey that's great!
Maybe it would be slightly more straightforward with as a "datalist textbox" object directly, with the additional parameter?
In my case, I have a list of about 200 000 items, so I would need a way to make ajax requests I guess, but the plugin is neat already for small lists
(If you're interested, it's an ini file over here, to which I'm making php requests like this for the moment - it's not really important that I add the feature, I suppose it would need advanced server side scripting too)