subclip.lua

-- copy to ~/.config/mpv/scripts/subclip.lua

local function subtext(name, data)
  if data == nil then
    return
  end

  local file = io.open("/tmp/subclip.txt", "w");
  file:write(data);
  file:close();
  os.execute("xclip -selection clipboard /tmp/subclip.txt")
end

mp.observe_property("sub-text", "string", subtext)