If i don't make a mistake, just record your track a first time without using the note variation slider, then attribut it to the parameter you want to change (attack, decay, filter.....) press the after button (just at the top of the slider) and record in overdub mod your sequence one again, while moving the slider. It should be ok this way, but as i'm not infront of the mpc right now, i'm not 100% sure

let me know if any problems, and i'll check that.