In the en_US.lang file, there is

tooltip.itemstages.name.default=Unfamiliar Item

But changing it (via resource loader/resource pack overriding) does not change the name of locked items within the game. Changing the other lines for the tooltips, however, do change them in game.

Is there a way to change the default name of locked items, or be able to in an update?

This tooltip key is no longer used by the mod. You can use setUnfamilarName to set a specific locked name for a specific item. I will look into making this changeable in a future update.

Thank you! Question, how is the script format laid out for that? I'm doing :

mods.ItemStages.setUnfamiliarName("Locked Item", minecraft:gravel);

As a test and it renames it back to gravel instead of Locked Item.

It should be mods.ItemStages.setUnfamiliarName("Locked Item", <minecraft:gravel>);