Another question from me :D, Is it possible to overwrite a config var on the fly? e.g. meltano run tap-postgres --config var=foo target-postgres

Yes, you will need to use environment variables to override configurations of a plugin: https://docs.meltano.com/guide/configuration#configuring-settings You can use

meltano config <plugin_name> list

to get some help with the names of the respective environment variables for a plugin.