Can Flutter reliably render "0.5"-width lines?

Yes, it is possible that the line might not be visible on low-density screens. Try for example to draw a 0.001 line on your device. It will disappear.

By using the devicePixelRatio you can prevent the line from becoming smaller than 1px.

import 'dart:math' as math;

final pixelPerPoint = MediaQuery.of(context).devicePixelRatio;
final onePixelSize = 1 / pixelPerPoint;
return Divider(
  thickness: math.max(0.01, onePixelSize),
);

/// The number of device pixels for each logical pixel. This number might not
/// be a power of two. Indeed, it might not even be an integer. For example,
/// the Nexus 6 has a device pixel ratio of 3.5.
final double devicePixelRatio;