The military has had a "top sling mount" kit that consisted of a strap that attached to the buttstock and looped over the top and a bent wire that went around the FSB to mount a standard GI "silent" sling.
It's been available since at least the 1970s. The front sling adapter I actually still kind of like - it can be made less noisy with the liberal application of 100MPH tape. The rear sling mount, on the other hand is practically useless, because it slides up and down the buttstock and never stays in a stable position - it needs another "leg" to keep it properly in place.
This what I used to do when I was issued M16s and with my M16A4 clone - use 550 cord and basically make the same thing, but with a tension "arm" higher up the sling that keeps it from shifting:
Something I learned in the military that was done a lot before the M4 was as big of a "thing." You can also use either this method or the issued top sling mount and wrap it in 100MPH tape, which was done often to keep it from shifting.
On top of these - many SOF folks "built" many of their own "tactical" slings using re-appropriated webbing. Popular components were padded machine gun slings, ALICE pack straps, HK MP5 3-point slings, and 2 QT. canteen general purpose straps. Partiularly using ALICE pack components gave you a "quick adjust" and "quick detach" sling that would become all the rage with modern tactical slings. Actually, if you take a machine gun padded sling - use an ALICE pack strap and sew new attachments in the ends - it's basically a VTAC or LaRue padded sling. Food for thought.
~Augee