I've written some python scripts that were meant for MultiBit HD that also creates bitcoinj wallets in protobuf format... It didn't extract the keys and just dumped out the raw protobuf. What was interesting to note was that the 12 word HD seed is actually available in plaintext in the dumped out data! So, I fired up gurnec's: decrypt_bitcoinj_seed and sure enough, it output the recovery seed!

You can then put the recovery seed into an offline copy of: https://iancoleman.github.io/bip39/
There are instructions at the bottom of the page for downloading etc

Put the seed into the "BIP39 Mnemonic" section at the top. Then click "BIP32" under Derivation Path, set "Client" to "Custom Derivation Path" and set the "BIP32 Derivation Path" to: m/0'/0


All your addresses/keys will be displayed at the bottom.



NOTE: You may need to click "show more" to see all your addresses if you have used more than 20 receive addresses... and you'll also need to set the "BIP32 Derivation Path" to: m/0'/1 to get access to your change addresses!