"I realize there is an argument against doing this with pre-tax dollars because then you're reducing your taxable income twice"
That's an internet myth. A repayment (be it a distribution or loan principal) is made by redepositing cash that you received. Cash is fungible, so the fact that you substituted one pile of cash for another is irrelevant, it's still fundamentally the same cash that you received. The whole concept of a repayment is that you are effectively putting back the same thing that you took out. New compensation that you use to make an elective deferral is not the same as money already in your savings which no longer has the characteristic of being compensation.
