test_const.lua

a = {}
a["__realdata"] = {}
a["__realdata"]["a"] = 1 -- save const a
a["__realdata"]["b"] = 2 -- save const b
local function const_index(o, key) return o["__realdata"][key] end
local function const_newindex(o, key, value) return end
setmetatable(a, {__index = const_index, __newindex = const_newindex})

print(a["a"])
print(a["b"])

a["a"] = 3
a["b"] = 4

print(a["a"])
print(a["b"])