Does butter really penetrate a steak when allowing it to rest after cooking?
I'm not sure the butter penetrates, but it tastes pretty good on the surface. ;)

I think it's pretty obvious that marbling plays a huge role in juiciness, so it stands to reason that if you bard or lard the meat it would also make a difference in juiciness (and indeed, the standard filet mignon with the bacon barding would seem to bear this out).

In the end though, it's always been my experience that once you cook the juice out of something, it's gone for good. I've always considered the idea of "resting" meat to be more about letting it cool to the point where the temperature isn't going to force out juice that would otherwise linger.