I’m pretty new to Modx, but I don’t think that this is (currently) possible. My working solution is to create a TV that contains the link for the start doc foloder and set the default value of it to an @INHERIT binding. So then I only have to set the link for the top most folder.
In other words:
template looks like this:
<div id="sidebar">
<ul>
<li id="dropmenu">
<ul><li>[*SubMenuHeader*]</li></ul>
[!DropMenu?levelLimit=2&startDoc=[[UltimateParent]]!]
</li>
<li id="feeds">
[*RecentArticles*]
</li>
</ul>
and my template variable called [*SubMenuHeader*] is a simple text variable with the default value set to the following:
@INHERIT <a href="index.php?id=15">Symposium</a>
Then I just change the template variable for each top level folder and leave it blank on the sub pages.
Hope that helps!
I would love to see this option built into DropMenu!