Patch here: https://github.com/tcatm/bitcoin/commit/126fdaef110aca47acf888a2e599167554280fe7
Branch multi-wallet: https://github.com/tcatm/bitcoin/tree/multi-wallet

It allows to select between multiple wallets within the datadir. e.g. bitcoin -wallet=foo.dat will load $DATADIR/foo.dat (or create it if it doesn't exist). Don't use it to access wallets outside the datadir.