While there's nothing in the rules about a salute, there are some signals we use on the lakes that are widely accepted as salutes.
We use one prolonged followed by two short blasts. In conditions where visibility is unrestricted, this signal has no official purpose, so we use it for unofficial purposes. It's how we greet other ships in our fleet, or if another freighter does us a favor somehow, or to just say hello to people ashore. There is a thriving community of shipping enthusiasts around the lakes. They call themselves boatnerds, and they love it when you blow a salute. Everybody seems to. In 9 years, the only complaint I've heard was from an engineer who was trying to sleep, but he was cranky anyway.
I've saluted coast guard cutters, and they'll return in kind. If it's illegal, then I guess it's a case of 'a crime committed by everyone, is no crime at all.'
I've only used it a few times aboard Wayfarer, though. It's impressive coming from a 730 foot laker with typhon whistles, not so much from a 29' plastic toy boat.