Towards a \ucshape and \textuc command for uppercase text (XeTeX, LuaTeX)

A full implementation in LuaTeX:

\documentclass{article}
\usepackage{fontspec}
\usepackage{xcolor}
\usepackage{luacode}
\usepackage{luatexbase}
\usepackage{luatexbase-attr}

\newluatexattribute\uppercaseattr


\begin{luacode*}
  local ucattr = luatexbase.attributes.uppercaseattr
  local GLYPH = node.id("glyph")
  local function makeuppercase(head)
      local orighead = head
      local string = unicode.utf8
      while head do
          if head.id == GLYPH then
              local att = node.has_attribute(head,ucattr)
              if att then
                  if head.char == 223 then -- ß
                      -- insert two 'S' glyphs
                      head.char = 83
                      orighead = node.insert_before(orighead,head,node.copy(head))
                  elseif head.char == 64258 then -- fl
                      head.char = 70 -- F
                      orighead = node.insert_before(orighead,head,node.copy(head))
                      head.char = 76 -- L
                  elseif head.char == 64256 then -- ff
                      head.char = 70 -- F
                      orighead = node.insert_before(orighead,head,node.copy(head))
                      head.char = 70 -- F
                  elseif head.char == 64257 then -- fi
                      head.char = 70 -- F
                      orighead = node.insert_before(orighead,head,node.copy(head))
                      head.char = 73 -- I
                  elseif head.char == 64259 then -- ffi
                      head.char = 70 -- F
                      orighead = node.insert_before(orighead,head,node.copy(head))
                      head.char = 70 -- F
                      orighead = node.insert_before(orighead,head,node.copy(head))
                      head.char = 73 -- I
                  elseif head.char == 64260 then -- ffl
                      head.char = 70 -- F
                      orighead = node.insert_before(orighead,head,node.copy(head))
                      head.char = 70 -- F
                      orighead = node.insert_before(orighead,head,node.copy(head))
                      head.char = 76 -- L
                  else
                      head.char = string.byte(string.upper(string.char(head.char)))
                  end
              end
          end
          head = head.next
      end
      return orighead
  end

  function makeuppercase_hbox(head,groupcode)
      local orighead = head
      if groupcode == "adjusted_hbox" or groupcode == "hbox" then
           makeuppercase(head)
      end
      return orighead
  end

  luatexbase.add_to_callback("hpack_filter",makeuppercase_hbox,"makeuppercasehbox")
  luatexbase.add_to_callback("pre_linebreak_filter",makeuppercase,"makeuppercase")
\end{luacode*}

\newcommand*\ucshape{\uppercaseattr=1}
\DeclareTextFontCommand{\textuc}{\ucshape}

\begin{document}
\hsize 7.2cm
\newcommand\sample{Draußen \i \ij\ ffl fluffiest fish \textit{König} \textcolor{blue}{àéîàáâãäåæ} çèéêëìíîï  ff \hbox{ðñòóôõö} ùúûüýþÿœš}


Lowercase {\ucshape \sample} Lowercase

Lowercase \textuc{\sample} Lowercase

% textcolor doesn't work in LaTeX's \MakeUppercase
% Lowercase \MakeUppercase{\sample} Lowercase


\end{document}

which yields

The program above just manipulates the nodelist iff the given attribute is set (to any value). Attributes are grouped, just like any TeX assignment.

It's a bit more complicated than I thought because we need to treat \hbox{}es separately.