pandoc --lua-filter filter.lua test.md -o test.docx

filter.lua

-- we use preloaded text to get a UTF-8 aware 'upper' function
local text = require('text')

function RawInline(el)
    if el.format == 'tex' then
        newcode = el.text:gsub('\\code', '')
        return pandoc.SmallCaps(newcode)
    end
end

test.md

title	test
author	baptiste
date	4/13/2019

title

This is \prolang{R}, this is \code{xtrfm()}.