Webform Conditional is not firing when using a hidden field
Following up on my response above, things like FAPI autocomplete_path and as your seeing webform conditional (i think) use the States API. Which is a limited version of scriptable javascript API from within Drupal7.

Drupal does ship with jQuery UI built in. And you can make direct usage of jQuery to alter/modify forms directly using drupal core components -- such as the bundle standard jquery packages that come with Drupal 7. Want a working jqueryui autocomplete example as an example -- here's 1 in Russian (Google translate works well here):

http://serho.ru/notes/raznoe/jquery-ui-autocomplete-widget-v-forme-drupal-7

I realize your request may be a non-coding module only solution; but such modules and APIs basically have limits as you've seen. There are pieces of drupal core that exist to let you homegrow your own solution.