Could a flat tax be more palatable with a generous exemption or like what has been suggested more, a tax credit/deduction to make it more progressive. That said, while it may sound painful, isn't the premise of a flat tax to encourage all taxpayers to be sensitive to government spending as well as reducing the influence of special interests who seek carve outs in the tax code? To be honest, I don't think it's so bad in theory to make sure everyone has a stake in something (such as having everyone pay for the safety net or etc). I understand the practical realities can be more painful though.
The idea that people generally make rational economic decisions is one of the most irrational ones economists have ever toyed with. For one thing, it assumes there are such things as objectively best economic policies. But more importantly, there's little evidence that the idea reflects how people actually make decisions. So the idea that a flat tax would be good because it efficiently conveys the cost of government to voters and thus can make rational decisions on government spending strikes me as completely ludicrous.