Every aggresive mob would get an extremely low chance of around 1% to drop a "fragment of mist" or something like that and when combined 5 with a diamond and 2 iron in the crafting table you could make a "mist shard".

The enchanting table then would get another slot next to where you put lapis, there you would put a mist shard and as you would enchant a book normally you can use such a shard to instead not use lapis nor any exp levels and enchant a book to a treasure enchantment.
It is a rare drop that you'd have to get 5 times anyways and its a diamong in crafting too so I think it's not anything TOO unfair or game breaking considering how un-explorable and tiny old minecraft structures are.

They aren't really a fun thing, these chests that can have enchanted books should be a reward for something but nothing awaits you in both desert and jungle temples.
That seems more unfair when you stumble upon one than this mod idea to obtain these books.