I think permitting individual posts to be shared with individual users would be great! However, I also think there are improvements to be made here that aren't nearly as complex. For instance, simply being able to allow/deny reposts on a post-by-post basis, make individual posts private (as in, followers only and not displayed to not-logged-in users) and so forth is very important.
Came here to request something like this! Livejournal friends-locking is another implementation of this concept, where "following"/"followers" is distinct from "friends", but otherwise it behaves similarly to the access lists OP described. Twitter circles is another, which behaves like a single access list.