Today I finally finished my rewrite of the MFRC522 library and sent a pull request to the upstream repository: https://github.com/miguelbalboa/rfid/pull/9. Now I look forward to hear Miguel's response - will he welcome my changes or reject them as "too invasive"?
Since last time I also added functions to use the MIFARE Classic Decrement/Increment/Restore/Transfer commands and an example of how to use them (it requires changing the sector trailer).
The next step in the project is building the electronics for driving a Ruko/Abloy EL582 solenoid lock I have bought for use in this project. I expect that to be a lot simpler than rewriting the MFRC522 library ;-)