I agree, it seems quite doable. The specific number proposed by FairTax.org for the "prebate" is relatively low, making the overall result somewhat but not very

I agree, it seems quite doable. The specific number proposed by FairTax.org for the "prebate" is relatively low, making the overall result somewhat but not very progressive (their proposal is a prebate equal to 23% of the national poverty line, e.g. $2650/yr for a single person with no kids). But that seems like a pretty simple issue to fix: just increase the amount, to whatever level of progressivity you'd like. If you increase it enough, you simplify the tax code and roll out something like a basic income at the same time. It's true you can't approximate every possible tax curve you might desire using the model of a flat sales tax rate + a flat prebate, but by varying the rate and the rebate size you can approximate enough curves for it to be interesting.