"The whole point of racing is to have fair racing, not to confuse the competitors." We all agree on that...

I've seen 2nd Sub and 3rd Sub used at different regattas to mean, more races coming.

Harry, I would expect anything this unusual to at least be covered at the skippers meeting. By the book, it needs to be in the SIs if you're using a different flag. I like Matt's view on the description of flag C in Race Signals, but as Matt and I stated, we would need to be careful about adding confusion, especially near a gate.

Especially at youth regattas, I always take a C, M, and S flag to the skippers meeting and hold them up one at a time, and ask, "What does this flag mean, and where would you expect to see it?" Usually a hush falls over the crowd, then I stare directly at the (paid) coaches...

Using L on the water and making sure everyone is on the same page is always a viable option, and is an excellent idea. There are some things to be careful about, such as being sure you don't say something that could be construed as a change to the SIs (unless the SIs speak to this), and generally being sure the players don't get in the way of finishers while trying to hear you speak.

Mike