pandoc.hs

    , Option "V" ["variable"]
                 (ReqArg
                  (\arg opt -> do
                     let (key,val) = case break (`elem` ":=") arg of
                                       (k,_:v) -> (k,v)
                                       (k,_)   -> (k,"true")
                     return opt{ optVariables = (key,val) : optVariables opt })
                  "KEY[:VALUE]")
                 ""

There's currently no way to set complex values like this from the
command line.  You'd have to embed them in the source file as YAML, or
in a separate file that you include on the command line. 

See https://groups.google.com/d/topic/pandoc-discuss/r-t0hXKSMTM/discussion