What you can do is to build a script on your server which returns the path with the string attached. So in your XML instead of
<streamUrl>http://streamingserverURL:1935/live/STREAMCHANNEL.sdp/playlist.m3u8</streamUrl>
you would have something like
<streamUrl>http://myserver.com/playlistX.mp4</streamUrl>
Then when they call playlistX.mp4 you process that with php and send their browser to the location
http://streamingserverURL:1935/live/STREAMCHANNEL.sdp/playlist.m3u8 + your encrypted string
aspiring