This site had a really simple execution of an expanding menu, using only css:
http://www.alistapart.com/d/horizdropdowns/barebones.htm
This could totally be implemented into the indexhibit template, however, the main difference is that indexhibit's front_index plugin formats menu lists differently.
If you check out the source of that site, you'll see that the 2nd level links are enclosed in a second ul. Whereas, the front_index plugin for indexhibit closes the section title AND links under the same ul. I'm tinkering with the plugin folder index.php file right now to see if I can tweak things to make this approach useful.
It's nice because it cuts jquery out of the picture and therefore allows for slideshow and other exhibit formats to work fine.