Hi,
I think you should look at scripting support for that kind of think.
But you might find that a bit painful for a simple feature like that.
Moving to a more powerful tool might be simpler. In particular,
liquidsoap (available on Debian and Ubuntu) does what you want very
naturally. Here's a liquidsoap script for your need:
output.icecast.vorbis(mount="blah.ogg",
switch([
({0h-12h},playlist("before.pls")),
({12h-0h},playlist("after.pls")),
({true},single("backup.ogg"))
]))
The third item is there to warranty that there is always something to
output in your stream. You could also remove it and use
playlist.safe() instead of playlist(), which triggers extra checks
(possibly costly).
Cheers,
--
David