I spent some time this week trying various sling mount configurations. I was unhappy with all but one.
Here are some photos and short descriptions of each.

Various sling mounts, rails and backing nuts.
First up was a sling swivel stud mounted to the backing nut form the Ergo grip rails Magpul sells. This is nice and clean setup, but I’m not crazy about these mounts:

Next was the idea from my post above. Using the same backer, I’ve a QD sling swivel/mount:

This mount sticks out far too much for my liking. I think this will likely snag on vests, clothing, other people, etc.
Compare its prominence to the first mount:

With respect to these two ideas, I don’t like having one stud/backing plate to support the front of my SHTF rifle’s sling. I would think that these could easily work their way loose over time. I would also imagine that the load could be excessive on the handguard using such a small mount.
I decided to look for mounts with wider flanges and with at least two mounting points. On the suggestion above by Rob, I ordered a Mesa Tactical HK style mount. When it arrived, I was dissapointed in that is was too large as well:

Moreover, the edges of this mount are too sharp and could easily slice one’s hand. That, of course, can easily be remedied by a Dremel.
After toying around with this mount (and punching a couple of extra holes), I think I’ve arrived at a satisfactory solution for me. What I did was to place a gradual bend the clip end of the mount and to insert the mount though the hanguard vent so the flange is inside of the HG. This will support the rifle even if the screws fail since the flange is wider than the HG slot. It also reduces the prominence quite a bit. I smoothed the edges and rounded the inside part of the flange where it meets the top part of the handguard.


Photo from rear of HG.

I decided to mount this toward the rear of the HG as much as possible to prevent rubbing on my meaty part of my palm/thumb. Carbine HGs are small!
There is enough clearance for the heat sheilds and they do not have to be modified:

I employed the same backing plates as before:

Finished install with Magpul screws:

Now I need to coat it with something. Not sure if this will be black or FG.