Can you do fully justified line and show both sides of the text frame?
You will see that it will stick out on both sides. And as there is no way to influence the distance to the text frame. So you can’t do much about this.
You could change the slanting height, but that would make the other side worse.
You can lower the x-height in the slanted masters. The left side is half the x-height.
Question is also what would a user expect. If you left-align the complete shape, the perceived relative position of the letters wanders towards the right. Not sure that is necessarily the better alternative.
I would do test runs with single italic words in an upright line.