void * your_custom_allocator(size_t size)
  {
      // Handle your locks.
      // Sanity checks, assertions, bounds checks, etc...

      void * result = g_your_buffer + g_position;

      // "Commit Memory" from your buffer.
      g_position += size;

      // Some more code...

      return result;
  }

  BASE64DECODE_DecodeEx(..., your_custom_allocator, your_custom_deallocator);

A guarantee that this "allocator function" is only ever called once.

Use whatever mechanism is available to you. Use a global condition variable, check it atomically every time you're entering your custom allocator, increment after a successful allocation. I don't know your system's constraints, nor should I...

If I pass in an allocator that returns the statically allocated buffer, then the second call to it must abort loudly.