BitcoinCoreWallet is only used when sendpayment is run with --rpcwallet. The user inputs their own coinjoin address and the wallet object only needs a change/internal address.
It's not possible to run a yield generator or tumbler from BitcoinCoreWallet so get_external_address() is never needed.
This wallet integration will probably be easier than Electrum or Mycelium, because it's already half-complete.
As @chris-belcher said in #368:
This wallet integration will probably be easier than Electrum or Mycelium, because it's already half-complete.