Context: I needed to call an extra callback during each step of an animation. This was so I could ensure that an expanding element was scrolled into view as much as possible. Doing it after the animation was already possible, but it looks cooler to also do it during each step of the animation.

I was able to add it easily with the following patch:

http://acko.net/dumpx/jquery.step.patch

There is already functionality in 1.0 alpha to pass extra parameters to .fx by passing a hash instead of a single callback function for 'callback'. After this patch, you can supply a step callback under the key 'step':

$(fieldset.contentWrapper).slideDown('medium',
  {
    // Make sure we open to height auto
    complete: function() {
      collapseScrollIntoView(fieldset);
      fieldset.animating = false;
    },
    // Scroll the fieldset into view
    step: function() {
      collapseScrollIntoView(fieldset);
    }
  }
);