The spending of all funds from whatever source saved or invested with after-(income) tax dollars should be exempt from NRST.
Now what government bureaucracy are you willing to allow to audit your books and and investigate your accounts and private holdings to establish the facts of whether or not a particular dollar you spend comes from after-(income) tax dollars?
Are you prepared to provide the government a full personal and real property inventory from which that can be determined?
I'm sure there could be some issues there, but at present I think that would be a good thing. I don't know how you could prove you had the money and that it was after tax, as opposed to borrowed money. I would think that would be necesary or people would borrow money and claim it was after-tax to apply for the credit, or even claim money they did not really have. I wonder if it was already considered and dismissed for some reason... even if just simplicity.