Does this support unordered list tags?

Certainly don’t see why not.
Well go ahead and put a quick demo together on the spacedonkey, but I guess you would just need to add a sub ul to the li. It might require roll your own method… but then again:

<ul>
<li>your thing with a [expand title="link" tag="li" elwraptag="ul"]this is the super cool extra info[/expand]</li>
<li>some other thing</li>
</ul>

Update: Nope… that does not work thanks to the less-than-awesome wpautop filter.
The roll-your-own method does, however, work:

<ul>
<li>your thing with a <span class="collapseomatic" id="linkone">link</span><ul><li id="target-linkone" class="collapseomatic_content">this is the super cool extra info</li></ul></li>
<li>some other thing</li>
</ul>

Demo:
http://spacedonkey.de/1481/collapse-o-matic-nested-ul/

Do let us know if you get this to work.