MapGuide / AIMS - WMS layer and background transparency and opacity

We set up a few new FDO WMS data sources and created layers accordingly. For an unknown reason for layers from some data sources the background was not transparent. All WMS layers are provided by the same organisation, we set up all data sources and layers the same way.
I finally found out that changing settings in "Set coordinate system overrides" for the data source and re-creating the layers solved the problem. I just changed image format from png to something else and unchecked the "Transparent" checkbox, saved the changes and then set the original values back again and saved it. 

WMS data source - cs overrides does the trick....

When creating a layer for the modified data source afterwards background transparency was present again. It is probably sufficient to just uncheck the "Transparent" checkbox but I haven't tested that - wasted enough time already...

After this issue was solved the second question came up - can we now set an opacity for the WMS layer? No, we can't... It is possible in AutoCAD Map to apply opacity to a raster layer but this does not work for raster layers in AIMS/MapGuide (at least not with FDO Raster Provider) and for WMS based layers.
Would be a nice enhancement if it were possible...

"Opacity" with Map raster layers - here you will find further information (German language),

Map, AIMS 2013