'Your hots now reduce incoming damage by 10%.' That would be sweet. So anyone with a rejuv or LB on them recieve 10% less damage. Would further encourage people to time hots for incoming damage and would be a specific reason to make sure you keep a hot on the tank.
Maybe only while in tree form the effect happens.
Dunno really, was just thinking out loud and wrote my thoughts!
Edit: I guess even simpler would be -' While Tree Form is active your party/raid recieve 10% less damage'.
So less damage reduction than other healers, but for a longer period of time.