I like the idea, but I would rather have it as multiple defensive points. Instead of one, central defensive platform, make it multiple unload points where the cargo is being unloaded.
Say, for instance, you blockade over one or two, and let the rest of them fall. You win, but you gain a substantially smaller War Chest. If you manage to unload cargo on all of the platforms, or retrieve all cargo and return it to the ones you have guarded, you get a slightly larger warchest, but no bonus for all of the platforms defended.
Say it's like this:
All platforms defended, all cargo delivered: +30% War Chest
All platforms defended, adequate cargo delivered: +10% warchest
All cargo delivered: Base War Chest
Adequate cargo delivered, platforms lost: -10% War Chest for each platform lost.