Rubocop allows generation of a TODO file, which is basically an extra config file that you include in the main one and that contains current violations per cop+file and sets appropriate values to cops that have numerical limits.

From there on you can only go one direction.

OP's ratchet would simply be a new custom cop that matches a string, and the whole ignore existing violations would Just Work (and actually "better" since one wouldn't be able to move the pattern around)