It sure is doable, so much so that it's done!
So, it only has 2 arguments (which are mandatory): timeout (in seconds) and message.
Example:logoffin.exe 120 "the computer will logoff in 2 minutes"
There's a minor quirk: it won't log off until the messagebox is closed. As a workaround, i added a timeout of 30 seconds to the messagebox, thus, the minimum time that it guarantees the computer will be logged off is 30 seconds.