I thought that in the past, if you created a hunt group, and had the first member of the hunt group be a key-system key of a unconnected ONS port, it would correctly light an MWI of that ONS port.
So my hunt group would be 200, with key-system key members 201-204. 201 is actually a ONS port. I could then create a mailbox for 200, which would light an MWI for 201.
(The kicker is that I tried to do this exact thing today, which I swear I've done in the past, and the MWI doesn't work correctly. It lights but when pressed doesn't go to the correct mailbox.)