I am not an expert here but I am thinking that this post should provide you with some clues.
I am struggling with the same thing, and have not found a solution for suppressing this warning. It makes reading the Console output difficult.
You can copy and paste the console output into Excel and then filter it to remove what you don't want to see
Debugging with Excel is a great idea, but unfortunately i'm not very familiar with Excel. Could you please provide an tutorial?
Dealing with this one right now. If you're using tail -f to follow the log, I would highly suggest using grep along side like so
tail -f flashlog.txt | grep -v "WARNING: For content targeting Flash Player version 14 or higher, ExternalInterface escapes strings using JSON conventions. To maintain compatibility, content published to earlier Flash Player versions continues to use the legacy escaping behavior."
This will at least remove that error message from the log.