Game data on cart is a read only chip, you can't flash it.
Save can be written, but you can't add things to the save that the game does not recognize, or it says save is corrupted.
Example:
Let's say items go from 000 to 100. If I edit the save adding an item with code 101, the game will give a glitch and say that the save is corrupted.
So even if they add the patch with new items you can't save on the cart, because the cart has to be played on any 3ds, even if they don't have a patch or internet connection.
We all knew that a patch to add new megas to XY was impossible. Well, at least those with some brain.