especially if it were printed vertically, i.e. you would be pulling the final product into your extruder in such a way that it would be pulling against the layer lines.

I agree that vertically-printed filament would have poor tensile strength, but isn’t most of the load from the extruder going to be compression, shoving it from the extruded gear to the melt zone and nozzle? Other than during retractions, doesn’t the tensile stress just comes from pulling the filament off the spool?

What about printing it in two halves the each have a flat bottom? Since the optical quality doesn’t matter, the line down the middle of the lens won’t matter.