"Defensive"? Dude, you have to be ever-ready!!
I have never gone for the very popular "they MUST have done it" argument, seductive though it is. It has been applied to any number of things, generally in order to support someone's preconceived notions/preferences (iron furniture on early guns is the big one :winking: ). If it is something you want to do, by all means, do so, but then if you ask "did they do it back then", the answer might be different (and undesireable!).
I have often wondered about the Great Vanishing American Rifle Sling. It is a rare German rifle indeed without sling swivels. There are several very early American rifles that also have sling swivels, continuing the tradition, but they seem to have all but disappeared by the beginning of the Revolution (and even before). They seem to have weaned themselves from the use of the sling, for whatever reasons. (as have I, and now, I don't miss a sling at all). If sling use had continued to be common, one would assume that rifles would continue to be made with sling swivels, which are simple to make and would cost a pittance to install. If you anticipated using a sling, why not "go for it" and simply get real sling swivels attached to the gun? Besides this, I have yet to see or hear of any documentation for the use of a "hasty sling", much less, the regular use of them. This is my reasoning behind my position.
Again, I have NO doubt that on occasion, a man might tie a string to his gun to quickly sling it across his back for some reason, like climbing over rocks or some such, where he needed both hands free. However, I think such a thing would be done using what materials were immediately at hand...such as a leather string ("whang", if you must :grin: ) from out of the repair kit. I don't think there were purpose-made hasty slings with barrel loops and butt-pockets, buckles or adjustment ties and such (in the 18th century, anyway. I have no inkling if such a thing might have been done in the mid- late 19th century...).