Some characters in Mating Flight use duodecimal. They are pretty emphatic about it -- they get annoyed when they discover that the natives of the new world have ten fingers because that means the wrong base, kind of thing. Duodecimal is a distinct point of pride with them.
Which means they use it everywhere, and powers of twelve too. Including metaphoric and approximate speech. So they'll say things like "A duodecade or two" not "a decade or two". I'm pretty happy with that.
The higher powers of twelve aren't so nice. scruff points out that twelve-cubed is a "great gross", though...
All actual numbers in the book are translated to decimal, by the way. Duodecimal only appears in approximate speech ("a dozen times that" vs. "ten times that"). Plus in multiples of things -- weeks are 12 days long, the dragons want a dozen times something as punative damages, etc. I'm not worried about most of this usage.
Consider "I doubt that they will forgive me this gross-year." vs. "I doubt that they will forgive me this century."
Here's what I recommend for the duodecimal equivalent of 'century'.
Consider And the ceiling was impressive and beautiful. Hovens had set it with thousands upon thousands of bits of mirror. vs. And the ceiling was impressive and beautiful. Hovens had set it with great-grosses upon great-grosses of bits of mirror.
Here's what I recommend for the duodecimal equivalent of 'thousand' in approximate speech.
Duodecimal, good idea at all?