Enter the config for single or multiplayer and change the 'seta com_maxfps' from the default 85 to raise the fps cap in the game.
bind W "+forward"
bind W "+forward;cg_fov XX"
bind W "+forward;cg_fov 90"
Now, everytime you walk forward, your desired FOV is applied; you will notice this once in every new level with your first step forward.
You don't need to set the config file to read-only.