Single Azimuthal Segment Texture Distortion for Earth TextureMaptoSphere

Quick Fix

FYI, a quick fix to the seam issue: just set the Start Theta to a small nonzero value such as 0.0001:

Note: my texture is inverted but the seam looks pretty good

vs.

Python Plugin

This would be incredibly easy/simple to implement as a Python plugin that would handle this by default. Not sure if this is the solution you are looking for though.