Thanks for the suggestion and we'll consider it for a future release.
Although they aren't notes in the typical sense, a workaround is to add a custom item to the order of service and put the notes in this, making sure the operator knows not to display them!
Also in our church, I've added some special "songs" to the database with titles such as "-sermon-", "-prayers-", etc. The "-" indicates to the operator to not display it.
Notes sounds like a neat idea.