There actually is a way to do this now... It might not be perfectly what you all want, but it does work!
MC supports and imports URL files. These are the "Internet Shortcut" files that IE and Firefox (and probably Opera too) create when you drag-drop the current address's Icon from the address bar over to the desktop. You can also manually create them by adding this text to a text file and saving it with a .url extension:
[InternetShortcut]
URL=http://yabb.jriver.com/interact/
(this one would obviously be a shortcut to Interact)
Anyhow, you can create these files for whatever web sites you want to be able to visit, and put them in a folder somewhere on your hard drive. Then import them into MC (you need to check the "Webpage Files" choice in the Import dialog's file types list of course). Then define a custom top level view scheme, and add [File Type]=[url] to the Step 4 search.
This will, of course, have them show up on the right pane, not in the tree itself, and they'll launch using your default browser (rather than inside MC)... But, it's a step in the right direction.
Oh, and you could get them to pseudo-show in the tree by adding [Album] (or some other custom field) to the View Scheme in Step 1, make sure "Populate Tree" is checked, and then copy the [Name] field over to the [Album] field. Then, they'll show up as children under your view scheme and you can double click on them in the tree to launch the link.