I think we're making too much of the concept of "rationalist fanfiction" altogether. A typical fan of MoR would presumably be interested in fiction delivering rationality-related ideas or emotional states; that might constitute (or at least overlap with) a genre, but it certainly doesn't imply derivative work. Neither is the bare fact of setting a work in another writer's world necessarily a genre label; about all you can reliably conclude from that is that the author wants to comment on the original's themes, or to explore implications the original author didn't. Fantasy stories are notoriously ripe with questionable implications, so we could expect the fanfic approach to produce some interesting-to-LW fruit, but it hardly seems like it should be promoted to a defining characteristic.
So, what does unify the set of stories we're talking about? Seems to me that it's their themes: empiricism, respect for the scientific method (as opposed to the trappings of science, Michael Crichton-style), and a belief in focused introspection. The use of an existing setting serves to play up the contrast between those themes and the assumptions of conventional genre literature, but it's not necessary, and the themes could work just as well without it.
Observation seems to bear this out; MoR shares a lot more, structurally and thematically, with The Cambrist and Lord Iron, or with Eliezer's earlier Sword of Good (both original works), than it does with, say, Harry Potter and the Nightmares of Futures Past. If we're actually looking at a nascent genre here, it's a subset of revisionist fantasy (other exemplars: Gaiman's Problem of Susan; Mieville's Bas-Lag books), not of fanfiction-as-genre -- although I'm pretty sure fanfic-as-genre is a lot smaller than it's usually given credit for, anyway.
(Granted, there might be a minority of potential readers who look at MoR or Luminosity's fanfiction.net address and immediately fixate on that as definitive, but that's not a rational response and I really doubt it's something we want to emphasize.)
I think you are right on with:
So, what does unify the set of stories we're talking about? Seems to me that it's their themes: empiricism, respect for the scientific method (as opposed to the trappings of science, Michael Crichton-style), and a belief in focused introspection. The use of an existing setting serves to play up the contrast between those themes and the assumptions of conventional genre literature, but it's not necessary, and the themes could work just as well without it.
But a thing I think unifies much of it (given that my four closest fri...
This may be old news to some people, especially the Russian speakers, but I didn't see an article about it here.
In 1999, Kirill Yeskov, a Russian paleontologist, wrote The Last Ringbearer, a 270-page take on Lord of the Rings from the point of view of a medic in Mordor's dying armies who is also a "skeptic and a rationalist." In fact, Mordor represents the forces of reason in this retelling of the story. As a Nazgúl (himself a former mathematician) explains, Mordor is "the little oasis of Reason in which your light-minded civilization had so comfortably nestled." Barad-dur is "that amazing city of alchemists and poets, mechanics and astronomers, philosophers and physicians, the heart of the only civilization in Middle-earth to bet on rational knowledge and bravely pitch its barely adolescent technology against ancient magic."
The story has been newly translated and is available in free PDF form -- in English and the original Russian. There's a recent review from Salon as well.