Although, if that were to be implimented FoD would be at a disadvantage since we have been concentrating at the planet level.
So far........

System Control should play a role in battles fought on the planets within the system. What about just expanding the 30% rule?
The way I look at the 30% rule is if a warzone falls within the 31%-69% range, that warzone is influenced/heavily influenced by the controlling side. If the warzone exceed this range, below 30%, or above 70%, that warzone is "owned" by the controlling side.
If a System Warzone is "owned" by either side, the 10% bonus it gives itself could also count for that side in battles fought on the planets in that system.