Unfortunately, don't have anything finished. Ended up passing thread_id of 0 for the tests i needed.
There is tbb::this_tbb_thread::get_id(), but it's not in a range we would expect it to be.

Thing is, in an ideal world the application code should not use thread_id for decision-making.
The only thing which really needs it is the texture nodes. The rest is our attempt to optimize pushing tasks from worker thread (which we don't need anymore if we go TBB). But even then it might easier to have something like:

static int global_thread_id_counter = 0;
...
__local int thread_id = -1;
if (thread_id == -1) {
  thread_id = atomic_fetch_and_add_uint64(&global_thread_id_counter, 1);
}

Here is the patch with my experiments from back in the days. Might be useful for something.