I have seen, flown and taught these manoeuvres and many more.
The point is more that a slow roll, done either horizontally or on a line, either way it should be flown ON the line, I do not see how a straight line can go ROUND anything - you can do this with a barrel roll or even a roll with some negative - but it will not be a slow roll and having -1 in the roll will not make it so, the nose should go up and down in the roll to stay on the line as you go from erect to inverted, but directionally is should stay straight.