"Hello HR, while working for the company I've used the company's computer to send a message to the company's other employees. But I think I own this message. Further, I think I can use this ownership to impose an additional contract upon the company's other employees, which they have to agree to in order to receive my work communications."

You should send it over Slack -- just make sure HR knows they also fall under your contract, at least that way they'll be laughing when they call you in for a chat.