After looking again, I think TurboTax does prompt for an explanation statement after entering the code 81 From 1099-R.
However, in looking again I've also found that the TurboTax developers have introduced a new bug for 2017, a bug not present in 2016 TurboTax. 2017 TurboTax is treating the entire gross amount of the code 81 distribution as subject to penalty, not just the taxable amount. I'll report this new bug.
