If bookmarks could be named/tagged at creation, that would work (just click OK to leave the name empty, then you know which bookmarked posts need to be revisited). Bookmark tags would also be handy for normal use so I would prefer that (improve existing feature, get two new benefits/uses) to adding Unread Post.
As admin I used to flag posts to read them later (very handy!), but that isn’t a good idea 