On further inspection, it looks like your blog is providing a different
solution: the ability to decide the sent mail folder at the time you
send the message. What I'm looking for is a way to have messages
automatically recognized (by a rule) as to which "sent mail" folder
they belong to.

What I'm stuck on is getting a script to appear as a "custom action" in
the Rules Wizard. I checked out that knowledge base article, but no
matter what I do in the VBA window (even saving and restarting
outlook), my new subroutines do not appear as custom actions. The
script I'm attempting to use as a custom action is very simple:

Sub MarkRead_CustomAction(Item As Outlook.MailItem)
Item.UnRead = False
Item.Save
End Sub

Any idea why it doesn't show up as a custom action?