I understand your thought process, and I am aware that there are applications where sights and slings are mounted to the gas block. However, I would not attach anything to the gas block that would add stress and potentially allow the gas block to lose alignment with the gas port. With that said, you can probably fabricate a small T-block type adapter, such that the slim stem portion of the "T" would fit into the recess in the gas block (for the current sling loop) where it could be pinned in place, and the bottom of the "T" head could be drilled and tapped so the sling stud can be threaded into it. This adapter can probably be formed by hand with small billet of mild steel or aluminum using a set of files or a small rotary tool/grinder.
