A more generic feature request would be to expose the Network Link DOM... that is,
the ability to do:

networkLink.getFeatures().getFirstChild()

and such. Does this satisfy the requirement?

Exposing the DOM for network links would be extremely useful too, but it wouldn't be
an ideal solution for this particular feature. You would still have to walk through
the DOM and call overlay.getColor().setA(n) on every individual GroundOverlay. That
could be slow for a very detailed superoverlay.

the.mutatron's approach would be perfect for this. For example, your root KML could
contain:

  <Style id="MySuperStyle">
    <IconStyle>
      <color>FFFFFFFF</color>
    </IconStyle>
  </Style>

and your superoverlay KMLs would contain:

  <GroundOverlay>
    <StyleUrl>http://my.example.com/my.kml#MySuperStyle</StyleUrl>
    ...
  </GroundOverlay>

Then you could get a reference to that <Style id="MySuperStyle"> element and simply
call style.getIconStyle().getColor().setA(n) and it would change the opacity of all
the ground overlays at once.

Agreed; setOpacity would be a more useful feature.

A closely related point: What we're really looking for is for this opacity setting to
be fast enough that we can animate it to do a fade effect. You can easily do that
with a ground overlay - you can get a smooth fade with a fast frame rate. My hope is
that making this single API call would set the opacity of all the ground overlays in
the superoverlay, fast enough that we can run it on an interval timer and fade the
entire superoverlay in or out, or even do a crossfade between two superoverlays.

Just ran into the situation where I'd need this feature. +1

Available as an experimental feature in GE Plugin 5.0.3506.3999 (API 1.003) as
KmlFeature.getOpacity and KmlFeature.setOpacity. More details here:

http://groups.google.com/group/google-earth-api-notify/browse_thread/thread/6a32fdfcc60236e