This is not much of a safety issue or it would likely have been changed in the code by now.
First off, only qualified persons should be working on an electrical system. Henry Homeowner isn't qualified & this trade is not a hobby.
If you are a qualified person, you should know all about multi-wire circuits & you shouldn't be working on them hot with the possible exception of taking a measurement.
What box would you require the 240 volt sign on? If it is a warning sign put on the panelboard, this might not be a bad idea, perhaps you could submit a proposal for the 2005 code cycle.
Few things are harder to put up with than the annoyance of a good example.