The board itself I haven't split, but I can imagine that it could be recreated on a few cards:
* Thread dial card
* Conversation point card
* 3 cards for the hostages
This is what I was thinking. The folks behind the brilliant dice game Ancient Terrible Things added cards in their expansion that allowed you to play the game without the board (which is a heck of a lot bigger than HN's), which they specifically said was for travelling.
I know it would lose some of it's look, but you could probably redesign the circular threat and simply make it a straight track on one card, or make it an oval. And the same would be true for the conversation track.
Your three cards for the hostages is exactly what came to mind as well. You could shrink that even more to one 'pool' card but it has an arrow pointing up and the top to signify you put the saved hostages above the card, and an arrow pointing down for those killed.