Seems unlikely that you could hear anything much quieter than your horn over the ambient noise. You could get a 12V piezo buzzer from the UK equivalent of Radio Shack and just try it for audibility. (mind you, there are commercial products so I guess they might work
web page)
Another way to go would be to get one of the signal cancellers that probably brent sells - they'll kill the signal after 30 seconds unless the brakes are on.
Or you could do what I did and develop a twitch in your left thumb to turn them off.
