not sure how easy this would be to, but would it be possible that a plugin running in the background could call a method within OSD to display a text message?
I was thinking with the email plugin, you can have it configured that every x minutes the email thread checks for new mail and then can send a message to the OSD if the user wants.
The weather plugin may be able to use it for storm warning alerts, etc.,
This is on the todo list, but it might not make it into the next release. I'm running a bit short of time.
Don't worry, its not an urgent thing,