How about keeping your mouth shut and flying the aeroplane according to the rules. If one has seen the other then it doesn't make any difference. You can continue with right of way if (if you have it) and take avoiding action if the other bod hasn't seen you. It's not dangerous because YOU'VE SEEN HIM!
And one wouldn't want to make an airprox report just because some-one is flying close behind you, that's their responsibility.
If you want a sign, rock your wings.
The best functioning fields I've visited are non-radio or A/G only.