I've created what is, for now, a suitable workaround, I believe.
All shortcuts - i.e. .desktop files in Xfce - are stored in /usr/share/applications and symlinked to the main applications menu.
I just used:
Code:
root@psychopig-xxxiv:/usr/share/applications# leafpad steam.desktop
to edit the Steam symlinked .desktop file and replaced
Code:
Execute=/usr/bin/steam %U
with
Code:
Execute=sh -c 'STEAM_FRAME_FORCE_CLOSE=1 STEAM_RUNTIME_PREFER_HOST_LIBRARIES=0 steam' %U
Now I can launch Steam from the main applications menu and the system tray icon appears.