I am not a specialist about Crestron, but you should be able to send a UDP message depending on the value of the slider.
Then you just need to have your fader to send this UDP message to the LPU-2: pb01/in=55
Where 01 can be changed to the index of the playback you wish to control, and 55 will be the value set via the fader.
Do not forget to use port 7000.
I hope this will help you,
Enjoy
