Is this a coin flip?

During some downtime at Hastings, a question was asked about methods of choosing a move. The question was "If you cannot decide between two moves, are you allowed to toss a coin to make a choice?"
The question was asked of a number of very experienced international arbiters (and myself), and we all pretty much said it would not be allowed as you were relying on external assistance. But being experienced arbiters we also suggested alternative methods of making such choices, including using the second hand on your watch to simulate a coin toss.
A couple of weeks later I found myself mentally tossing such a coin at the Gibraltar Masters. I knew my opponent played a specific line against 1.e4 and was pretty sure I was going to reach the diagrammed position. The choice was either to castle, or sacrifice on f7. Having looked at it further it turns out that the sacrifice is not quite sound, but there was still a wrinkle. On two occasions previous opponents had sacrificed on f7 with a 1-1 result. The odd thing was that where he chose the correct line, he eventually lost, but when he chose the losing line, he actually won. So the sacrifice may have worked, if for the wrong reasons.
So when I reached this position at the board, I spent 5 minutes deciding what to do. To sac or not to sac. Eventually I ....