Henry Beard and Douglas Kenney, who actually wrote the thing, were founders of the National Lampoon after they left Harvard University.
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It would certainly be tough to make a 9 hour trilogy out of Bored of the Rings. };-}
Many of the jokes refer to product names and TV ads from the sixties, or Earlier. Serutan (natures spelled backwards) was a laxative product that went defunct (so to speak) in the 60's. Such could be updated, I suppose but it's hard to imagine a modern scriptwriter quite capturing the brilliance of the original.
As Harebrain said, much would be lost without the narrator's asides.
There are certainly no shortage of visual gags. It would probably need to be CGI. It might be tough to convince a herd of Merinos to cooperate as War Steeds.