In my testing, virtualbox virtual machines would only display transparent windows with alpha if the accelerated 3D drivers are installed. I was hoping that checking for compositing would help (see r6575), sadly it does not. (we already check rgba visuals, etc..) Hopefully there is away to detect the lack of transparency, one I am not aware of.
niche setup, not worth the extra effort.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/577