One key to reach real-time requirements is not to prevent Lua from dynamically allocating the stack (or any other object), but to provide an allocator function capable of providing memory in a fixed time. (Others are fine tuning garbage collection, preloading libraries and replacing/not using the io library after initializing, by the way).
Growing an object requires copying, which can't be done in constant time. Most CPUs can copy very quickly but it's still an O(n) operation, however efficient you make the memory management.
Some operations performed by the garbage collector may also not meet hard realtime requirements. It all depends on how much delay you can handle.
