Considering that Sands of Time are used to convert ilvl100 Tome equipment into iLvl110 equipment, I rather doubt it'll drop in Coil 2. Unless it was a drop that went directly into your inventory without lotting, it'd render the loot tables for Coil 2 a complete mess.
I think it's more likely that it'll be rewarded once a week for completing certain Trials. Perhaps beating all Extreme Primals + King Moogle, or something along these lines.
Honestly, I suspect there must be some surprises we haven't seen yet. There are a number of items that should be absurdly valuable, and we have no idea where they drop. The only thing that makes sense is that there are Fates, Trials, Boss Fights, and repeatable quests we haven't yet heard about.
Or that we are once again, going to have to down Primals repeatedly for "Special Barter Currency."