one of the issues is that having your currency added to coinbase gives it huge legitimacy. every altcoin would love to be on there.

The most simple way for coinbase to distribute the coins would be if coinbase "listed" the forked coin. They would also be compelled to do this when other coins airdrop based on holdings of BTC (eg. Stellar and Byteball). This would set a precedent resulting in large numbers of ropey forks and altcoins trying to do drops to push coinbase to list them.