A small library that will let you use luajit cdata how you would hope.

pointer.lua

local ffi=require"ffi"

local topointer 
local pointer_mt
local key_address , key_ctype , key_cdata , key_methods = {},{},{},{}

local function ispointer ( ob )
	return getmetatable ( ob ) == pointer_mt
end
local function denormalise ( ob )
	if ispointer ( ob ) then
		return ob[key_cdata]
	else
		return ob
	end
end
local function normalise ( ob )
	if type ( ob ) == "cdata" then
		return topointer ( ob )
	else
		return ob
	end
end
pointer_mt = {
	__tostring = function ( o )
		return string.format ( "%s: 0x%08x" , tostring ( o[key_ctype] ):match("<.->") , o[key_address] )
	end ;
	__index = function ( o , k )
		local m = o[key_methods]
		if m then
			local v = m [ k ]
			if v then return v end
		end
		return normalise ( o[key_cdata] [ k ] )
	end ;
	__newindex = function ( o , k , v )
		o[key_cdata] [ k ] = denormalise ( v )
	end ;
	
	__eq = function ( a , b ) return a[key_address] == b[key_address] end ;
	__lt = function ( a , b ) return a[key_cdata] < b[key_cdata] end ;
	__len = function ( a ) return #a[key_cdata] end ;
	
	__add = function ( a , b ) return normalise ( denormalise ( a ) + denormalise ( b ) ) end ;
	__sub = function ( a , b ) return normalise ( denormalise ( a ) - denormalise ( b ) ) end ;
	__mul = function ( a , b ) return normalise ( denormalise ( a ) * denormalise ( b ) ) end ;
	__div = function ( a , b ) return normalise ( denormalise ( a ) / denormalise ( b ) ) end ;
	__mod = function ( a , b ) return normalise ( denormalise ( a ) % denormalise ( b ) ) end ;
	__pow = function ( a , b ) return normalise ( denormalise ( a ) ^ denormalise ( b ) ) end ;
	__unm = function ( a , b ) return normalise ( - denormalise ( b ) ) end ;
	
	__tonumber = function ( o ) return tonumber ( o[key_cdata] ) end ;
}
local pointer_cache = setmetatable ( { } , { mode = "kv" } )
topointer = function ( cdata , methods )
	assert ( type ( cdata ) == "cdata" , "Not cdata" )
	local address = tonumber ( ffi.cast ( "intptr_t" , cdata ) )
	local ctype = ffi.typeof ( cdata )
	
	local v = pointer_cache [ address ]
	
	if v then
		assert ( tostring ( v[key_ctype] ) == tostring ( ctype ) , "Differing ctype" )
	else
		v = setmetatable ( {
				[key_address] = address ;
				[key_ctype] = ctype ;
				[key_cdata] = cdata ;
				[key_methods] = methods ;
			} , pointer_mt )
		pointer_cache [ address ] = v
	end
	return v
end

return {
	topointer = topointer ;
	new = function ( ct , methods )
		return topointer ( ffi.new ( ct .. "*" ,  ffi.new ( ct ) ) , methods )
	end ;
	change_methods = function ( pointer , methods )
		assert ( ispointer ( pointer ) )
		pointer[key_methods] = methods
		
		return pointer
	end
}