I'm looking forward to seeing this issue resolved.

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This would be very welcome. Any estimate on this?

I would love to see this issue taken care of. 

Any idea when this might be added? Changes in 3.7 seemed to have broken my workaround for dynamically changing the image opacity.

I'll add my vote to allow a the ImageMapType to easily set the opacity value!  It would be a shame to have to redo what I have, using ImageMapType, using a custom MapType, just so I can dynamically change the opacity.

I would like this as well.  Any current workarounds?

Yeah, Had to ditch the Image Map Type and use this guys technique of creating tiles in <img> tags.  He made a class called MCustomTileLayer you can find a working example here: http://maps.forum.nu/v3/gm_customTiles_N1.html.  Made a few tweaks for what I was working with, but this worked great for me.

Another workaround can also be found here:

http://www.gavinharriss.com/code/opacity-control

This worked really well too.

Thanks for the links!  It also seems that I can simply remove and add the ImageMapType again with the desired functionality, or use setAt(index,options).  Would this be bad on performance?

wmsOptions.opacity = new Val; //wmsOptions is global
wmsMapType = new google.maps.ImageMapType(wmsOptions);
map.overlayMapTypes.setAt(0, wmsMapType);

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This was marked as 'fixed' in comment 12 above... is there now a way to dynamically change the overlay opacity using ImageMapType, without resorting to using a custom map type?

Yep, you can now set opacity on an ImageMapType like this:

YourImgTiles.setOpacity(newValue);

NewValue can be passed from a slider or a constant that you set.