Yes, this is possible using server-side playlists and the server object
model to manipulate the playlist on the fly.

1.  Create a unicast broacast publishing point that sources from the
following serverside playlist.  Name the pub point Test.  Modify the
encoder_ad.wmv element's src to point to your infomercial/Watch for the
upcoming stream file.

<?wsx version="1.0"?>
<smil>
   <excl repeatCount="indefinite">
       <media src="encoder_ad.wmv" repeatCount="indefinite" begin="0"/>
       <media src="lpp://LiveEncoder1" begin="SwitchToBroadcast"
id="liveElement" />
       <media src="YouMissedTheShow.wmv" begin="liveElement.end"
repeatCount="indefinite" />
   </excl>
</smil>

2.  Create a separate broadcast publishing point that sources from the
encoder.  Call this pub point "LiveEncoder1".  In your case, you are
pushing from encoder to server so you would simply push to a pub point
named LiveEncoder1.

3.  Save the following vbscript as "StartEncoderRemotely.vbs"

Const BCPPName = "Test"
'create server object
Set WMServer = CreateObject ("WMSServer.Server", "machineName" )
'set bcpp object
Set BCPP = WMServer.Publishingpoints(BCPPName)
'set sspl object
Set Playlist = BCPP.SharedPlaylist
'fire event
Playlist.fireEvent "SwitchToBroadcast"

4.  Edit StartEncoderRemotely.vbs in notepad and modify the machineName
with the machine name of the WMServer

5.  Start the Test pub point (without starting the encoder), connect a
client and notice how the ad continuously repeats.

6.  Start the Encoder session, start the LiveEncoder1 pub point, then run
StartEncoderRemotely.vbs from a cmd prompt on the server machine.  note:
this vb script can also be run on the encoder as long as the user has admin
access.

7.  notice how the playlist interupts the ad and jumps to the liveEncoder1
element.

note:  You can run the VB Script from the encoder as long as the user has
admin access to the server.  See the MMC admin help docs regarding dcom
access.  

Thanks for using Windows Media Technologies.

Jorge Novillo [MS]
Digital Media Division

This posting is provided "AS IS" with no warranties, and confers no rights.
The use of any included script samples are subject to the terms specified at
http://www.microsoft.com/info/cpyright.htm