If anybody's looking for a replacement for BlendMode.ERASE on non-flash platforms, I just figured out that you can erase with whatever color you want and follow it up with a threshold() call

FlxSpriteUtil.drawPolygon(_canvas, poly, 0xFF80684E, null, {blendMode:BlendMode.ERASE});

#if !flash
// BlendMode.ERASE is not supported by non-flash targets, so we manually erase the pixels
// with a threshold() call
_canvas.pixels.threshold(_canvas.pixels, _canvas.pixels.rect, new Point(0, 0), "==", 0xFF80684E);
#end