It is possible to change texture of any item using this path:
.minecraft\resourcepacks\\assets\minecraft\mcpatcher\cit
But it is not possible to change the item's texture if a specific player is holding specific item. My suggestion is to add this option. Either allow it by typing in players name or their UUID.
Let me start with that link. https://bitbucket.org/prupe/mcpatcher/wiki/Custom_Item_Textures
It is possible to change texture of any item using this path: .minecraft\resourcepacks\\assets\minecraft\mcpatcher\cit
But it is not possible to change the item's texture if a specific player is holding specific item. My suggestion is to add this option. Either allow it by typing in players name or their UUID.
(just an example of how would syntax look)