Insofar as the idea of a living wage depends on knowing what the real costs are for real people, any such system would be subject to error as it is generally more difficult to measure the real costs facing people, than it is to know their income. The idea that immediately lept into my head was that this could be mitigated by the provision of a packaged living wage lifestyle.
But on the other hand credit/debit/etc payments might be collecting very good information on what people are spending their money on, with specific regional granularity. Yikes, it starts to feel a little big brother.