!Fov on cj

I assume it uses SetClientDvar combined with cg_fovscale.
Like solving all programming problems, break it down into small steps first, for this example first let the server listen for the player's input of !fov and then decide what to do next.
Here's a useful resource ZeRoY's Call Of Duty Script Reference (also if you haven't already looked - Modding and Servers).