Outlook 2010 Navigation Pane - Show Unread Count on Mailbox
Solution 1:
One "sort-of" solution is to first disable unread count in all folders, as described in:
Enable or disable Folder Unread count in Subscribed folders in MS Outlook 2010.
To get back the unread count, you would then create a Search Folder that aggregates all the emails of the folder and all its subfolders into a single view, modifying the Search Folder criteria to show only the unread items in that folder.
The following article might also be useful :
Refreshing unread counts for IMAP folders in Outlook 2007 or 2010