We have some persistent mysql storage happening with simple hashes (comma-separated lists ["key,value"])

This is a huge code smell. You should not have to delimit or otherwise worry about the storage of your data. That's what the database is for. A better approach might be to have two separate columns in the table instead of just one. You could name them key and value. Then you wouldn't have to worry about efficiently storing the pair -- the database engine would worry about it for you.