Where <SLIDERVARNAME> is the CL slider var name. "ONSLD<SLIDERVARNAME>" is executed everytime you adjust the CL slider.
You can then tell your script to read the raw value of <SLIDERVARNAME> and evaluate it w/in the script.
EDIT: If you're just creating a custom VOLUME slider, the raw slider value can be evaluated w/in rr w/o the need of an external script:
Skin Custom Slider:
I think that might work...*shrugs*