Probably the greatest idea I ever had in this week - a VirtualBox VM launcher right inside a Fluxbox menu !⸮
All menu items will launch either VBoxManage, the rdesktop-vrdp/rdesktop client or the headless front-end so it's really not as interactive as you would expect. Script that will generate the (sub)menu file can be found here.
After downloading, place it in a folder and execute it after setting the permission bits. Your menu file will be found at ~/.fluxbox/VBoxMenu . Include this in your main menu file and you are done.