I would suggest through a dedicated variable tag <nbt>, that uses property and indexer notation.

Examples:

<nbt>player.ForgeCaps.thaumcraft:warp.warp[2]</nbt>
<nbt>chestplate.Damage<nbt>

The last example is already attainable via existing variables, yes. I meant it more as to demonstrate an alias to that inventory slot.

This is certainly possible, but one of the main concerns about this would be the data access speed. If it turns out to take too long to retrieve the data this might not be easily possible.

There are a few other things to consider as well. This tag would be very useful for many different things. Items that the player is wearing/holding, player data, in world block (mouse over) and possibly others.

Because I am considering a rewrite for the sake of getting a cleaner API implemented I might change the way some of the function tags currently work in order to unify everything into fewer tags overall.

I would suggest having a secondary update speed, then, for NBT checks, configurable in case using the standard update speed is too slow. For jetpacks, in particular, an update every 4 seconds may not be ideal, but it would certainly be enough to avoid running out of fuel.

Might be fine though, since it doesn't run on the rendering event.