Heat oven to 400°F. Pierce potatoes; roast on lined sheet 50 to 60 minutes until fully soft. Cool 10 minutes; split and scoop flesh.
Reduce oven to 350°F; butter 2-quart dish. Mash flesh with melted filling butter, milk, filling sugar, zest, and 1/2 teaspoon salt. Cool until warm, not hot.
Beat in eggs until combined; spread filling in dish.
Mix pecans, oats, flour, topping sugar, and remaining salt. Rub in cold topping butter to small clumps; scatter evenly over filling.
Bake 30 to 35 minutes until center 160°F and topping crisp and browned. Rest 10 minutes before scooping.