It's an impossibility for a couple of reasons...

1) The market would become flooded with them, and prices would drop way down to almost nothing as there are not that many buyers who would want to buy them.

2) Because of this, the price on the level 26 helms and level 31 armors would also drop to almost nothing, so people would get annoyed if they looted one from a locked crate.

3) Because there is an achievement for crafting/upgrading the mythic armor and helm, they cannot be removed from locked crates or else new players would not be able to get this achievement.

For those reasons, it's better left untradable.