I've just managed to successfully create a program that can convert the historical*.odb files to CSV and another that converts CSV files back into odb format. I've tested the programs, making minor changes to the database, and they work beautifully so far. Nevertheless, I should probably do some more rigorous testing before releasing it to the public (message me if you'd like to help in testing, preferably if you already have a custom version of the lahman database you'd like to implement and you're comfortable with command line tools).
Edit: I've recently merged these tools into a single program with a simple gui. You can find it here:
OOTP 21 Database Converter