I'm not sure if you know what getItemIcon does, or I'm just not understanding you correctly. It returns the icon that is displayed in your hand. Normally it uses the texture for the item you are holding, but while pulling back the bow, it must be changed to the textures with the string being pulled back. If I were to return a different texture from the item, it would also change the texture in the hot bar, which is not what I want to do.
As a new user of Forge and a relatively inexperienced programmer, what you are saying concerning getFOVMultiplier is beyond my knowledge. I've read a bit about reflection in order to better understand it, but I'm not sure if I would be able to make a hook. I'll look into it. I should be able to add a simple hook to EntityPlayer for getItemIcon, unless you think I shouldn't. I'll use that as a sort of introductory course to creating Forge hooks, then make an attempt at changing the FOV with a hook. Do you know of any tutorials for creating and submitting a Forge hook? Thanks for the help.