I am experimenting with a filter that augments the plain text output of Pandoc and I am using RawInline quite often. One surprising thing I found out was that you can never make them render with the plain output.
Here is an example filter:

return {
  {
    Code = function(elem)
      return pandoc.RawInline('plain', 'balls')
    end,

    CodeBlock = function(elem)
      return pandoc.RawBlock('plain', 'doodoo')
    end
  }
}

If you run it with pandoc --verbose you get messages like

[INFO] Not rendering RawInline (Format "plain") "balls"
[INFO] Not rendering RawBlock (Format "plain") "doodoo"

Is this on purpose? If yes, can it be changed?