There is no official answer that I have found, but I asked [the same question on BGG][1] about Betrayal Legacy, which has the same rule for starting the haunt, without ever getting an answer.

However, this is in all likelihood only a theoretical question, and not a practical one, because the odds of it coming up are pretty tiny. On the 12th role, you have just about a 1% chance of not getting 6 or higher. And that's after having failed the other roles. The total probability:

Chance of not starting haunt on role #:
-  #1: 1
-  #2: 1
-  #3: .96
-  #4: .81
-  #5: .60
-  #6: .40
-  #7: .25
-  #8: .14
-  #9: .08
-  #10: .04
-  #11: .02
-  #12: .01

The odds of all of these events happening in a real game? 

0.000000385464%, or 1 out of every 259,427,843 games.

Now if you're limiting yourself to only using 8 dice, since the rules aren't clear about what to do if you don't start the haunt after drawing your 8th omen, then the odds are a better, but still unlikely.

  [1]: https://boardgamegeek.com/thread/2750608/2-questions-about-haunt-rolls