Hi,
There isn't a way to mark a message as unread in Messenger Express. This feature was added to UWC (and I assume upgrading to 6 and deploying UWC isn't what you had in mind).
The way that you can do it would be to use IMAP and proxyauth into the customers account to change the read flag. To do this would require writing some kind of script (e.g. the perl module IMAPClient support proxyauth) and changing the flag -> I leave that as an exercise for the reader.
So in a nutshell there is no 'easy' way to do it.
Regards,
Shane.