I would be able to create an EMF image (programmatically or from a software) having the background color 50% transparent and the other elements completely opaques.
I'm already be able to create a background completely transparent, but not with a certain alpha/opacity.
Creating this semi-transparent background would allow to overlay the image to another background without completely hide it.
Does anyone know if it's possible with the Enhanced Metafile format?
The answer is no: EMF (or WMF) image format supports full transparency but not opacity between 0.0 and 1.0 because it doesn't support colors with alpha channel.