On a regular A2 stock, there is a loop at the bottom rear (toe) of the stock.

The MOE stock has cutouts to loop a sling through, but these are oriented differently (the cutouts are "vertical" instead of "horizontal" like on an A2 stock).

Is there any kind of adapter that can thread through one of the cutouts on the MOE stock and act as a "horizontal" attachment point like the sling loop on A2 stock?

I've included pics to show what I'm talking about.

I don't know of any.  Probably isn't any because it would make an overall better way to loop a sling worse.

Or just use one of these: http://www.cheaperthandirt.com/product/2-UM2520-0

Not that I know of. I run a VTAC sling on my MOE stock and just used the "vertical" loop to attach it.  I dont have too much trigger time since I have set it up but I dont have any complaints right now.

Pull a piece of paracord through that hole at the toe of the stock and tie it into a loop using a double fishermans knot.  Thread sling around paracord loop.  Or, if you want something a little stronger, a piece of aircraft cable and a brass crimp.