If she were a dictator, I'd fear for my life, because her tax system is no better than any other provided in history. Yes, indeed, it steamlines the current tax system and erradicates many issues that were generated by private interests (Child deduction f.e.), but it doesn't strike at the core of the problem.
First of all, a libertarian should always be wary of progressive tax system (here coined as a negative tax), because it is inherently unfair. It taxes wealthier people harder than "poor" people and this even exponentially.
Let's examine her claim. We have a flat tax system which constitutes of a mere 15% on the income of every citizen. If the citizen had 10000 euro per year, it'd be 1500 euro. A wealthier person with 100000 euro per year would have to pay 15000 euro. This is a linear increase and the fairest possible solution, because regardless of success and other characteristica, all tax payers are taxed equally (in the right sense of the word).
Now, let's examine her idea. Let's say there is one worker with 10000 euro per year, he won't be taxed, he even would get some REFUND from the state (which is anti-thetic to any free market economy or liberal agenda). Meanwhile a 100000 euro per year income individual would have to pay up to 35% taxes, which would constitute 35000 euro. Is this fair? Why would he like to live in the US and being ripped off, while he could work in Dubya for free?
But there is even more to this, because the gradient of the tax percentage rise can be pretty unfair. Let's assume that you have to pay 15% up to a salary of 50000 euro and all above 16%.
When you have a salary of 50100 you are dumped (it'd be 8016 in taxes). While a salary of 50000 would provide only 7500 euro in taxes. You'd lose about 400 euro to the state and you wouldn't work any longer than before, I assume.
If this isn't unfair, I don't know what Jane thinks is unfair and anti-liberal.
I think, the incentive is exactly opposite to what Jane thinks would happen. If I were a worker who had to decide between paying taxes and not paying taxes, I'd choose the latter. With some luck, you'd fall into the category of REFUNDs, which is even more sweet.
So, if you have the choice between a job with 28500 dollar and a job with a salary of 25000 dollar, which one would you take?
Ahh, yes, this plan is perfect if you want to kill incentive and productive work, it is nothing new or even special, because similar ideas have been tried in the US for decades and they didn't work.
These ideas might steamline the state's fiscal service, but it wouldn't be fair or liberal in the libertarian sense of the word. It's just another statist ideology to put the masses at rest and destroy liberty in a country that has already suffered from the loss of liberty.
I think those plans are falling into the same category as Milton Friedman's pathetic but very successful strategy to tax income at the employer level rather than at individual level.