I would make a schedule (see Schedule in the manual http://docs.playitsoftware.com/PlayItLive/201/PlayItLive.pdf )
Assuming you want some kind of track rotation for the rest of the day but your 1 hour MP3s to go out at say 10am, make a generic "clock" for the other 23 hours and schedule that for 23 hours.
In the one hour you want the MP3 to play out schedule a different clock that plays (only) your file.
If you make the file name suitable but generic, My Programme.MP3 for example, then if you put todays programme in the right place named My Programme.MP3 then I don't think PlayIt Live will mind that the file has changed (but I haven't tried this).
There may be an early fade problem if the programme is longer than the first one.