This is luckily no longer the case as reddit user mr_irony has put together quite the list. In spreadsheet form you can view which Legendary crafting materials drop in which Act. Even down to the waypoint.
The list seems to be evolving as well, so if you have anything to add please do so in the comments section below. Now time get hunting. I need me some rare mats.
Act Waypoint Bounty Legendary Drop