I got some time to do a simple calculation with the mortality tables at
Actuarial Life Table
Basically, I modeled a series of payments to a population of 50-year-old men for 30 years. According to the table, about half of them will die sometime during the period. Each year I would only pay the ones still living. I took this series of outlays to NPV with an arbitrary interest rate (I picked 5%). Then I also took the PV of the constant payment over 30 years assuming everyone lived. It turns out the "mortality discount" in this case is about 13%.
So assuming no cost of money difference between the mortgage and the annuity, I could buy my mortgage payments for life and still have 13% of the payoff still in my portfolio.
I think this effect is interesting enough that I might consider keeping my mortgage for a few years and annuitize later if interest rates rise, (or pay it off anyway if we get a few bullish years of growth).