Well, actually - it seems like the only possibility would be another CMC 5 creature being destroyed at the same time. Somehow that has to give us, probably by some indirect means, a bunch of hyperstage tokens. Something to ponder anyway.
If we can't get that to work, I do like the idea of using Cabal Surgeon to retrieve Child of Alara and/or Forked-Branch Garami, to soak up a usage of Engineered Explosives. Although I also like the idea of Aethermage's Touch freeing up the ability to target Child of Alara. So lots of ideas, nothing near completion yet.
Edit: Meh - since the destruction ability of Child of Alara doesn't happen immediately, it doesn't really work as a way to prevent us from returning Engineered Explosives to our hand. Oh well.
Edit: I think that what Iijil said about a tap chain being bad may not be a problem. Yes, with two Coastal Drakes, X Skyshroud Archers can create 2X-1 Drake tokens - but X can only be 1, so we get 1 Drake from 1 Skyshroud Archer token. So I think it can work.
That being said, how about the following: Use Doomed Necromancer to bring Boggart Mob back from the dead. Use Giant Oyster, coupled with Devout Chaplain, to target Doomed Necromancer. We can have two Doomed Necromancers, so that they can bring back the two megastage creatures, and then a Giant Oyster can destroy one, bounce the other, and they both can be brought back to the battlefield.
How do we get Giant Oyster back? I was thinking two Giant Oysters. If we destroy two Giant Oysters, then we get two tokens, and they can be used to generate three Doomed Necromancer tokens. But, the two Giant Oysters are now in the graveyard, so if we want to repeat the process, we need to use two of the Doomed Necromancer tokens to bring back the two Giant Oysters. Net benefit: One Doomed Necromancer token at the cost of one Engineered Explosives, as desired.
This seems ripe for abuse, but I'm hoping it somehow works.
Edit: Okay, another attempt.
Have at it!
Edit: I forgot to add a way to get Reveillark back. Adding Crushing Vines and another Reveillark should do it, although there may be a more efficient way to restore our creatures.