At line ~694:
        incoming['__user_name__'] = user_name
+       if job.history and job.history.id:
+           incoming['__history_id__'] = job.history.id
+       else:
+           incoming['__history_id__'] = 'unknown'
I have tested this change and it appears to give me exactly what I want.  My question: does this change appear correct

Yes, the change is correct.

and can it be incorporated into the main galaxy code-base?

Can you provide a usage scenario for including history_id in the tool dict?

Thanks,
J.