The MAD Hookup has been out of stock for a long while now, I use to sell them. Per their facebook page they were/are having production issues.
https://www.facebook.com/MissionAdaptedDesign
The mount itself is very well done and rock solid. There is nothing like it out there that will clamp on to the receiver extension with a large single loop, that I know of. The closest thing I could find with a quick search is from Midwest Industries, but it does not have a single loop like the MAD Hookup.
Midwest Industries: End Plate Sling Adapters
OP mentioned earlier concerns about QD mounts possibly failing, in my opinion, as long as you buy quality QDs and mounts it is not very likely you will have one fail. Also I would not be concerned about using a sling mount end plate damaging the threads on the receiver extension. I have tried out a variety of different mounts to find one that worked for me and ultimately went with a QD end plate. If you do decide to try out a sling mount end plate and still do not want QD, I would take a serious look at the IKH SLAP plate. That was the previous sling mount I was using, and I just threaded the sling through it instead of using a mash or hk style hook.
IKICKHIPPIES Home of the SLAP