I believe this is not possible using just the Out of Office Assistant. This was done to stop infinite OOO message loops, bringing Exchange servers to their knees in the event of two people having OOO messages set, with a message being sent from one to the other.
It can be done however, using Message Rules, but this is not advisable (for the aforementioned reason).
Ok, found a dirty hack that will allow you to do what you requested - however, it is a really dirty hack.
You'd need to customise the script for an individual user (if you have many users, they need one each). Place the script on the Exchange server, and Schedule the script to run once a day. The reason this works is because each time you re-enable the OOA, it wipes the list of people it has sent OOO messages to. You would of course need to remember to remove the script when the user returns to work, otherwise everyday it would re-enable their OOA.
Also, change line 3 and 4 for your environment, and the user in question.
Set objMAPISession = CreateObject(”MAPI.Session”)
strExchangeSvr = "svr-exch-01.yourdomain.loc"
strMailbox = "jane.doe"
strMAPI = strExchangeSvr & vbLf & strMailbox
On error Resume Next
objMAPISession.Logon “”, “”, False, True, 0, False, strMAPI
If err <> 0 Then
Wscript.Echo “An Error occured: ” & err.description
strOOOMessage = "Jane is having babies and will be absent for a while. Please try again after October. Ciao"
objMAPISession.OutOfOffice = False
objMAPISession.OutOfOfficeText = strOOOMessage
objMAPISession.OutOfOffice = True
strOOOMessage = objMAPISession.OutOfOfficeText
Set objMAPISession = Nothing
Please note: I have not tested this. You will need to deploy it with a test Mailbox, and do some testing with it.
Wow, I need a shower after that.