Transparent future
Not being able to set my mind to doing anything useful, I played a little with rgba windows. For a future version of Xfce that can depend on cairo-based versions of Gtk, it should be possible to only make the panel background transparent, while keeping text and icons fully opaque.
Just changing the panel window and the base plugin classes, I got the following result:
Not too bad. All widgets that have their own X window, or that do some custome drawing are not transparent, but the others are. I had to choose a theme with white foreground text to make labels somewhat readable. That will require special attention to get right. Shadows or outlines maybe…