Perhaps a crazy idea.. perhaps not...
At least while we have the current mechanic, why not add a fee for players to join port battles....?
When buying the flag, the player has the choice to log as a clan purchase or individual purchase.
When choosing Clan purchase, the player sets a price for non-clan members to be able to join the fight, be it a percentage of the flag cost or a token amount.. or perhaps even the ability to lockout non-members..
When choosing as an individual purchase allow the player to be able to choose a cost per player that cannot exceed the cost of the flag, or any lesser amount. (Edit, the total cost charged for players cannot exceed the flag cost.. ie: flag/25)
What do you think?