How to make hidden NBT strings in MM items?

I'm trying to add a hidden NBT string to my MM items for upgrades. There's a /upgrade command that takes the item you have, removes it, and runs the command to give you the next one. It's an alright process, but we have to upgrade the script each time we add items that can be upgraded or change names for streamlining. I'm wondering if we can add a normally unreadable NBT string to help the script read from a YAML file instead of putting the item names in the command sequence.

I'm hoping we...

How to make hidden NBT strings in MM items?