I've also living with this dirty workaround:
1. Remove the gnome-panel entry within gome-session kicking sequence
In /usr/share/gnome-session/sessions/gnome-flashback.session, Those components would be defined like this:
So remove this red part to avoid kicking gnome-panel within session init.
2. Add autostart desktop file for gnome-panel with some "Delay"
Something like this named 'gnome-panel.desktop' in $HOME/.config/autostart/:
This could work for my flashback session...
I suppose gnome-setting-daemon in trusty might need a bit more time to load their plugins, or indicators-applet would need some tricks to load into gnome-panel?