Here's an idea worth discussing:
Change the campaign buffs outside Cyrodiil to be an average of the four normal campaigns.
Yes, when in Cyrodiil, the buffs of a campaign should only apply to players in that campaign. Outside Cyrodiil, all players receive buffs that are an average of the four normal campaigns. So, your home campaign only accounts for 1/4 of your faction's buff in PvE land.
If all players are affected--whether they play in Cyrodiil or not--they will have a vested interest in the events there. I feel this will encourage more players to join their faction's cause.
I also feel that this would encourage more players to go to the current low-population campaigns. After all, if you can gather enough players, you might be able to change the color of all the keeps to your faction's color. Thus, benefiting everyone in your faction.
"It is not the critic who counts: not the man who points out how the strong man stumbles or where the doer of deeds could have done better. The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood, who strives valiantly, who errs and comes up short again and again, because there is no effort without error or shortcoming, but who knows the great enthusiasms, the great devotions, who spends himself for a worthy cause; who, at the best, knows, in the end, the triumph of high achievement, and who, at the worst, if he fails, at least he fails while daring greatly, so that his place shall never be with those cold and timid souls who knew neither victory nor defeat."
Teddy Roosevelt, Paris, 1910