Conceptually, this isn’t much different than some of the stuff Romney was proposing. 401(k) and similar retirement savings vehicles represent enormous tax preferences, and limiting such preferences with respect to the highest earners is similar to Romney’s proposals to raise revenues without hiking tax rates by limiting the use of certain deductions by those with higher incomes.
What’s pernicious about this proposal is the amount of tax information and reporting that would be required. Citizens for the first time would have to provide coordinated information on an annual basis about total retirement savings. The extreme degree to which this infringes on privacy, especially in relation to the relatively small amount of revenue raised, make this a non-starter.