I think you should formally write to Apple and give notice of 30 days to contact you or you will reveal it.
Send it to the USA media and regulator too
Send it to the USA media and regulator too
I am guessing you haven't tried that excuse on the users your witholding is leaving exposed.
Do you believe the mitigation would be difficult to engineer? If, say somebody else, publicly disclosed the unmasking technique how long would you guess it would take Apple to implement a verifiable fix?