I can see how a pillow would work, unless you simply slip off it when moving around - I tend to move around a lot.
I've never slept in a hammock, so I'm only theorising here. I'm looking at maybe getting one, though, which is where my interest comes from on the subject.
Thinking about it further, I'm wondering if just shortening one of the end ropes would do it rather than hanging a pack or having a second rope. If the shorter rope was tied lower it might throw the centre of gravity - the place the hammock would sag most, further up towards the head end - supposing you slept with feet at the short end and head towards the long end. In my mind, I see it creating a better sleeping position. Of course, that's only in my mind.
I have actually got a hammock, though just a cheap net one. Unfortunately, I have no trees in my garden, so can;t try it out. I might go down the woods, just to see, though. It'd be an interesting experiment.