mt.lua

function shallow_copy(t)
  local t2 = {}
  for k,v in pairs(t) do
    t2[k] = v
  end
  return t2
end

local function make_observable(t, key, prev_keys)
  if type(t) ~= "table" then
    return
  end

  local prev_keys = prev_keys or {}
  local _data = {}

  if key then
    table.insert(prev_keys, key)
  end

  for k, v in pairs(t) do
    make_observable(v, k, shallow_copy(prev_keys))
  end

  setmetatable(t, {
    __index = function(self, key)
      return _data[key]
    end,

    __newindex = function(self, key, value)
      if type(value) == "table" then
        make_observable(value, key, shallow_copy(prev_keys))
      end
      _data[key] = value

      path = table.concat(prev_keys, ".")
      if path ~= '' then
        path = path .. '.'
      end
      path = path .. key

      print(path .. " changed!")
    end
  })
end

local data = {}
make_observable(data)
data.one = { boo = 5 }
data.blob = { ding = { dong = {} } }
data.blob.ding.dong.hey = 'hi'

output

$ lua5.1 mt.lua 
one changed!
blob changed!
blob.ding.dong.hey changed!