-- Replace "char1", etc., with the clefs' codepoints.
local clefs = {
  -- [char1] = true,
  -- [char2] = true,
  -- ...
  }

local function getnodes (n)
  -- Set start and stop to the first and last nodes composing the
  -- sublist associated with the clef (e.g. spacing, associated
  -- glyph...). Here the default code sets the clef as the only such
  -- node.
  local start, stop = n, n
  return start, stop
end

local function leaderize (head)
  local n = head
  while n do
    if n.id == node.id("glyph") and clefs[n.char] then
      -- Detach the node(s) from the list.
      local start, stop = getnodes (n)
      local prev, next = start.prev, stop.next
      start.prev, stop.next = nil, nil
      -- Create a box containing the clef.
      local b = node.hpack(start)
      -- Create a cleader whose width is the same as the box.
      local l = node.new(node.id("glue"))
      l.subtype, l.leader = 101, b
       -- A glue's dimensions are held in a spec node.
      l.spec = node.new(node.id("glue_spec"))
      l.spec.width = b.width
      -- Insert the leader node in the list.
      l.next = next
      if next then next.prev = l end
      if prev then
        l.prev, prev.next = prev, l
      else
        head = l
      end
      n = next
    else
      n = n.next
    end
  end
  return head
end

callback.register("pre_linebreak_filter", leaderize)