I suppose you could, after Starting Tier, continue to assign a d6 as an exploder but roll it in excess of your actual roll and use it to determine exploding.
Example-
You attack with a 4d8 pool. You roll 4d8 + 1d6. The rolls come back as :
d8 = 2, 3, 6, 7
d6 = 6
You exploded. So now you...