If a player is connected to Squeezenetwork, have it send some sort of SlimProto packet advertising itself to the server it was last connected to but alerting the server that it's connected to Squeezenetwork.

The seed of this idea came from Dan Evans.  Brilliant.  Wouldn't waste much bandwidth, and it would allay massive user confusion about why they can play internet radio but can't play songs on their hard disk...

Then instead of saying 'Player Not Found' you could have a message like:

'Your player is currently detected on the network, but is connected to Squeezenetwork and not to this SlimServer.  For more information click here' and then have a page with more information on what the difference is and why they would want to be on whichever server they are / want to be on..