I was looking for a way to vary the rate of a step based effect using the fader on my Ion and managed to pull it off using a simple strobe effect, but I believe any intensity effect will work using this simple trick (VS 2.0.1.9.xxx).
I created a simple on/off step effect with an on state of 100 off state of 50 and assigned it to a sub. Running the fader up to full and running the effect produced the desired result. Bringing the fader down lowered the intensity BUT ALSO SLOWED THE EFFECT! Well now we're half way there. You can slow the effect but no one will know because they can't see it.
I then went back to the actual effect and changed the on state from 100 to (wait for it) 1,000. Yep, the board took the value, which of course is irrelevant because you can't light a fixture past 100%, however the on state won't dim until the fader is brought down to around 10%. The off state still follows the traditional convention of dimming with the fader but that's no big deal either. Simply create an effect that runs stupidly fast (beyond any drummers ability hit the kick drum) and set you fader to the speed you want.
This isn't perfect mind you. A tap will be right on the money every time, however you can preset this during sound check and tweak during the show if needed.
Addendum: Entry must fade by Size and Rate
[edited by: zel at 9:35 AM (GMT -6) on Thu, Aug 15 2013]