Okay, it finally works. The last piece of the puzzle was the @Register directive. This page helped me figure that out: http://www.vbinfozine.com/a_aspnet_derived_ctl.shtml
Here are the steps I took:
1. Create a new class that inherits the control you want (modeled after the menu class in the OrbitOne example posted above)
I just included this as part of my CSSFriendly project, and within the CSSFriendly namespace.
2. Modify the *.browser file to point to the new class (i.e., CSSFriendly.Whatever)
3. To use the control on a page, the @Register directive looks like this:
<%@ Register TagPrefix = "tag"
Namespace = "CSSFriendly"
Assembly = "CSSFriendly" %>
Namespace is the namespace in which your new control resides. Assembly is whatever *.dll contains the code. In my case, since I made my control part of the CSSFriendly project, both of these are CSSFriendly. I'm happy to provide any assistance if you decide you want to use this solution (you probably won't have any problems with it.. I feel like the @Register problem I had was pretty amateurish, but I'm a C++ programmer by nature

).