Go to a gun show... I always see a couple people walking around with an AR pistol on a sling. Of course, the cop at the door had to clear the pistols before they brought it in, so they didn't have any problem with it.
Anyways, I've never heard of it being considered any kind of mod. It's just a sling. Some larger revolvers used for hunting have a sling attached as well, and this has been going on for years.
A sling does not make a pistol a rifle, both my Draco and my TEC-9 have sling mounts on them; and both are still classified as pistols. Perfectly legal...