Metafiction is self-conscious about language, literary form, storytelling, and directly or indirectly draw attention to their status as artefacts
(From Wikipedia's description of metafiction.)
I'd say Lord of the Rings fulfills this.
Self consciousness about language:
Tolkien was a philologist. His thing was constructing languages. To begin with, each species or group of people generally has their own distinctive language, which fits in with that species' characteristics, and also follows normal language patterns (i.e., the languages are realistic). A list of languages mentioned LotR (not complete):
- the Elven languages, such as Quenya
- the dwarvish language
- the language of the Rohirrim
- the common speech, Westron
- the Black Speech
- the language(s) of the orcs
And so on and so forth. Further, names also fit in with culture and language - think Ghan-buri-ghan vs. Elrond vs. Nazgul vs. Khazad-dum.
The form of Lord of the Rings is that of an epic. It's meant to emulate stories like Beowulf, or the Norse sagas, or the Finnish Kalevala, or the Oedipus cycle. Further, it is presented as such: in the foreword, Tolkien describes translating and transcribing the story he is about to tell from the Red Book of Westmarch.
A not insignificant portion of the Lord of the Rings is tales from the past, or reminiscing of the way things were. Think of the story told in the council of Elrond of how the ring came to be, and how it was passed to Frodo, or the story of Luthien and Beren told in the hollow of Weathertop, or the song sung by Gimli in Moria, or the song sung by Legolas just inside of Lothlorien, or the stories told by the Ents of the Entwives, or the reference of the barrow-wights to the past kings that lay there, or...
The list clearly goes on.
Further, there's some meta references - in the foreword, Tolkien mentions how he got the story, through the Red Book of Westmarch, which was copied and past down, and in LotR it's described how the Red Book was written, first by Bilbo, then Frodo, and finally Sam. There's also the appendices.
Status as artefacts
First, see above about the Red Book of Westmarch and the appendices. Second, things like the image provided, showing the inscription on Balin's tomb in the Chamber of Mazurbal act as "artefacts". He also describes hobbits in the foreword, and the family trees, almost as a historian or an archaeologist might. See also the section on the literary form of the work, and how LotR comes across as a true epic.
It seems that the definition of metafiction can be awfully confusing. Some definitions define metafiction as drawing attention the fictionality of the work - breaking the fourth wall, so to speak. Others define it as presenting the fiction as artefacts. Let me quote a paper on The Lord of the Rings' status as metafiction:
Tolkien also set out to reproduce that singular effect of which he speaks, the effect of the work reaching us as an echo of an echo (of an echo …) from a remote antiquity, expending his art in increasing the distance between the (mostly) Modern English text the reader would be holding in his or her hands and the fictional characters and events of which it told. For this purpose, he integrated his major works of fiction into an intricate metafictional structure, presenting them within their fiction precisely as such echoes of echoes: translations of redactions of ancient works, telling of things even more ancient.
(From The Books of Lost Tales: Tolkien as Metafictionist.)
His work rather clearly fits the second sort of definition, but whether it fits the first sort of definition is debatable. Does The Lord of the Rings draw attention to its status as fiction? I'd argue it draws attention to its status as something other than a normal book; the foreword and the continued references to the Red Book of Westmarch do that.
Perhaps, though, the scenes in LotR where we see Bilbo, Frodo, and Sam writing in the Red Book of Westmarch indicates that this that we are reading is a story, albeit a "true" one. So perhaps this fits the first type of definition?