I just realized I can disable the 'uGui Image (Script)' on the slider handle, then make any custom game object a child of the handle, so now I can detect and apply any old playmaker action to that. This is real good because I wanted to use iTween animations on the slider handle too. After a couple days of messing with this I'm a little surprised at how simple the fix is, but it's even better than I expected because it gives me lots more control than I was hoping for.