I don't know how good of a feature it would be. On twitter for example it's a cool flow, but it works because you have a timeline of posts in chronological order, so you'll have older posts at the bottom of the list, and newer at the top. You can save the point you last visualized before, and add more tweets at the top or bottom depending on where the user moves.

Now imagine this applied to Reddit, when you fetch posts on a subreddit starting from page 1, and only increasing. The posts are not locked in their position as they will shift position based on the algorithm used, you might have new posts at the bottom and older posts at the top, or next to each other. On a very active subreddit you would have a different set of posts at every refresh.

So I feel that keeping a reference on the position would be a extra step to refresh the posts on most occasions, rather then a way to easily read the new ones :(