A plain LuaTeX example for this would be

\directlua{
  callback.register('kerning', node.kerning)

  font.current(font.define {
    name = "dummy",
    type = "virtual",
    characters = {
      {
        width = 655360,
        height = 655360,
        depth = 655360,
        commands = {},
      },
      right_boundary = {
        width = 655360,
        height = 655360,
        depth = 655360,
      },
    },
  })
}
\char1
\bye