You where there and I was not. But that is different from what you said earlier when you said 'on reflection' you could have braked and swerved. Difficult I know when split second decisions have to be made. Another road user (the car driver) committed a 'wrong' and you either faced a pragmatic course of action or an ego driven action.
The pragmatic decision would be to keep both hand on the handle bars, brake and move in to keep yourself away from a ton of steel capable of killing or crippling you. Get home safe and sound that evening, kiss the missis, and carry on with your life
The ego driven action would be to continue on your line, get close to a ton of steel capable of killing or crippling you, take a hand off your handlebars therefore making you slightly more unstable and show your displeasure and slap the car. Well you probably will get home safe and sound to kiss the missis (fingers crossed), and you probably will carry on with your life....hopefully.
Lets not dress up 'car slapping' as some sort of road safety device, its an ego driven means of showing dis-pleasure at another road user mis-deads. If I was faced with a situation of such danger I wouldn't be car slapping I would be car punching with all my strength and screaming at the top of my voice. You have to be honest with yourself, if you could have braked and swerved to avoid it then do it, you owe that to yourself and your family.