Long, long ago... I used to work without benefit of a "peg," a "stand," a "cradle," or whatever you want to call the hoof stands and props that we commonly use.
Whether you're using your own leg or a stand, the mechanics are about the same when you're working on a front limb. On a hind limb, however, the mechanics work a bit differently...
When you work a hind "in your lap" or on your thigh, you tend to abduct the limb (pulling it outward, away from the midline) more than you do when working it on a stand. Likewise, more often than not, you tend to work the limb higher than you would on a stand.
So... if your horse has scar issues & tissues, working a hind leg in this manner might--especially because of the abduction--cause some instability, pain, irritation, or whatever. It's certainly worth trying a different approach and seeing if it makes a difference.....