I haven't done extensive testing of active property dialogs, but I did try it out with custom parametric shapes and modifiers. Very cool. It's great not having to press OK all the time to see new results while I was playing with PHUR.
I'm a little concerned on the ASL side though. Large, possibly taxing parametric shapes that take a long time to generate will become a pain to deal with if it takes several seconds to several minutes every time a value is changed. I think it would be nice if there were a way to flag a parametric shape as non-active via ASL, or have a checkbox in the parametric shape's dialog where if checked, it becomes "active", and by default it'd be "active" but in ASL the coder has the option of changing the default for that shape to "non-active". Maybe as a parameter in the #plugin directive?
Are layers intended not to be part of the active activity?