I'm planning to translate the code like that:
m[i, j] => __index(m, i, j)
m[i, j] = v => __newindex(m, i, j, v)
Beware that translating multiple assignments might be trickier than it seems, if right-hand-side terms and/or __newindex methods have side effects:
foo, bar[i,j], bar[k,l] = f1(), f2()