The lifters themselves if rotating properly would be cylindrical with the potential for some taper to the top and or bottom as they wear

To achieve what you are suggesting, you would have to coat just the ovaled part of the bore

Clearance wise even if the bores are ok in "width" but worn on the "thrust" parts of the bore - it would wear in an X or hourglass shape in a direction modified by the twist of the lifter as it rotates, a very complex 3D shape to be sure