James, the problem with regarding non votes as protest votes is that you can't distinguish between apathy and protest.
It would be more effective to have an additional box on a voting slip that allows you to protest against the available candidates. Then at least you would get a truer picture of how people felt. those that can't even be bothered to turn out to protest are the apathetic portion of the electorate!
Perhaps politicians should then not only start to evaluate the protest votes but also why so many people can't even be bothered to turn out to protest. Maybe if there was protest box the electoral turn out would go up!
We can but hope.