You didn't disguise your alterior motive very well, wry. It doesn't seem your interested in a fair tax policy so much your interested in making sure people don't have more wealth than you believe is okay. The a flat tax would do a lot more than just make wealthy peopel wealthier (not really sure how that's a bad thing). First and foremost, it would be fair. Everybody pays the same percentage of tax on any type of income.
To claim that libertarians aren't for freedom if there okay with rich people having more money is simply false. One person having more money than another does not inherently mean the person with less is less free. Money is the means by which many freedoms are exercised for sure, but it is not societies or the government job to make sure that everyone has enough money to do so. It's your own job to make sure you have the money needed to do the things you want.
If you had complete control over the myriod factors that affect your life and well being, then yes, you could at least make that arguement. But you do not. That's why we (through government) must take a proactive stance in creating the sort of society we want, or else someone else will do it for us, and that will probably be the biggest tiger in the jungle.
You cannot control labour laws, interest rates, trade and tariffs, immigration policy, energy use, environmental protection, civil rights, or many other things as and individual. That's why power, in the form of large scale wealth, should not be concentrated in the hands of a few of the most voracious businessmen, but used for what we deem necessary and appropriate. Exxon and Microsoft are not going to do that for you.