I want to do the same thing when holding a particular item (eg a stick), but it will not work with the same code. How can I do the above behavior?
You could also try sending a custom resource pack with a retextured fishing rod and a custom translation for the fish button's text
You cannot do this currently, MCPE has this hardcoded to a couple items and entities. The best you could do would be to either use a resource pack and retexture the items or to spawn an invisible entity in the middle of the screen and set the interactive tag when hovering over it.