I was just reading the effect of Big Burn and don't believe you can chain it to BLS-EotE, not because the summoning removal is a cost but because you can't attivate BB until a card targeting monsters in the graveyard is ACTIVATED, and Envoy is not activated until the removal is completed. Plus with the strict working the rulings are based on now, BB reads ...specifically designates (a) target. I believe Envoy targets two monsters. I could be wrong.