Using a KmlLayer object within the Google Maps API, is it possible to control the opacity of a KML's image overlay features ?
Looks like it takes KML, sets a 50% opacity (static) http://www.geocodezip.com/geoxml3_test/v3_geoxml3_KML_samples_zoom_false.html slider could control the opacity setting
Slider control (not KML) but impressive
This transparency / opacity control for Google Maps API v3 might be adaptable to your needs:
Disclosure: I developed the control.