My usual approach in a heavy cap (when my opposite number on the opposing team is defeated, or never shows up at all) is to stay back a bit and offer support, such as bots and bats, a watchful eye for new targets, and the occasional fire support when an enemy fighter or bomber starts making threatening gestures towards me.
To me, that's not at all unfair, since I'm not really engaging directly in the battle unless the enemy chooses to involve me.
EDIT: That having been said, I do try to stay within 3k of the fight, though I announce that I'm offering support to my allies, rather than engaging in the battle personally.