[Feature] Flag command to work on Held Items

[QAngelusQ]

As far as I know the only way to flag an item is to use the itemtag.with_items using either the give command and drop command.

I want to flag an item the player is currently holding, but the flag command only works with entities / server. Would it be possible to add a flag command for itemtag?

[mcmonkey4eva]

More reasonably: A way to quick update items in-place.
An ItemTag cannot be modified so directly, as it does not know where it came from. It's just a blob of data that you can manipulate through tags, and then push onto where it's meant to go.
A command to replace an item at a specific spot, with equivalent spot-reading tags, would be relevant.
Also tags to find an item's slot as needed.