Further fiddling about: Goodreads widgets can be added to sidebar but they tend to become distorted due to space allocation. LibraryThing widgets can be added but only the LibraryThing link displays. A shame because, to me, LibraryThing widgets are preferable ... they have a changing display that rotates through one's entire library while the Goodreads widgets have a fixed display that will always only contain whatever books fit the parameters when you set it up (it'll only change if you manually redo and re-add it after your Goodreads account has been updated). LibraryThing's "animation" coding is presumably the reason for their nondisplay ... most likely more than Dreamwidth can handle at this time.
Sideline: Speaking of LibraryThing and Goodreads, I'm planning to take a look at the BookLikes site sometime soon. Site runs slow (for me anyway and it might be a holidays thing), but it does have the attraction of not having any Amazon tentacles in it (Amazon owns Goodreads and now a piece of LibraryThing as well); currently completely independent and free unlimited book space. And who knows, it might have Dreamwidth/LJ-compatible widgets. ;p