I answered my own question.
Yes, after-tax contributions are deductible from your income.
You take the deduction using form 8889 and you get the deduction for after-tax contributions on line 25 of your 1040.
However, this only seems to be available if you are itemizing your deductions. The current standard deduction is $24,400 for married filing jointly so this means that I would need to somehow haver enough deductions to exceed that in order to make contributing to the HSA worth it. Otherwise, I might as well put it in a normal savings account without any restrictions.
