MEH92,
I did exactly what you are describing with the sling swivel. I drilled a hole slightly lower than the centerline of the buffer tube (just below the transition of the flat side into the curve of the top), tapped it and screwed the swivel post in. No problem. CAUTION - REMOVE THE BUFFER TUBE BEFORE YOU DO THIS!!! Don't ask me how I know this - ARGH!!!
The thing that I didn't like was I had to make custom shims to go under the post in order to make sure it was in the correct orientation once it was tightened down.
I really can't comment on how well it will hold up during operations but, considering all of the other swivel holes are made this way I wouldn't expect any problems.
Sorry I can't send pics; my digital camera is out being repaired (damn software gremlins!).
EDIT - You can leave the rubber pad off; you just have to use the countersunk screw that is included instead of the regular screw to hold it to the buffer tube. The area under the rubber plate is anodized just like the rest of the part with grooves cut into the surface; I assume for grip. Shouldn't be difficult to make your own plate or even just cover the surface with skateboard tape.
M590 Man