Also Looking/Wanting that feature!
For whats it's worth...
I remember someone writing a script (back in Xfce 4.6 or 4.4 era) that would check the windows displayed (widow buttons display I believe) and if blank/nul changed the 'show panel' setting to on. Auto started and with a timer (checked every second or 2) so behaved liked deamon.
Only bad thing was windows not covering whole sreen (like calculator) also deactivated panel!
More work but I simulate the effect by making launchers on desktop and also make a auto-hide panel that copies those launchers and placement.