geoserver - Setting transparency in an SLD file for a 4 channel GEOTIFF

I'm struggling to set the a background colour to transparent in an SLD file for a 4-channel GEOTIFF using GeoServer 2.1.4.

I think I've set my background colour to transparent alpha layer in a graphics package correctly.

I've set up an SLD file to read Red/Green/Blue channels of my GEOTIFF = this resolves colours correctly for the RGB image = but still shows the background black colour (see below).

When I do a GetFeatureInfo request on the WMS, I can see an ALPHA_BAND=0 where I want the transparency and ALPHA_BAND =255 where I do not want transparency (see attached figure).

I was hopeing to do something like setting the ALPHA_BAND=0 to Opacity=0 in the SLD or something. However, there's no option in SLD schema that I can see ?

I've read the SLD cookbook and the SLD reference documentation. Alpha transparency is referred to but only with ColorMap' property. Can I mix and match 'ColorMap' and ChannelSelection' properties in the SLD ?

Am I going about this the right way ? Thanks in advance for assistance!

This is the SLD I'm using:

    
          xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="http://www.opengis.net/sld http://schemas.opengis.net/sld/1.0.0/StyledLayerDescriptor.xsd">
      
            bathy example
            
              RGB image 3 channels

              
              
                    
                      
                            
                              
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