You can use JoyToKey to get incremental FOV/Zoom in CoD:
http://www.electracode.com/4/joy2key...%20Version.htm
First set up the buttons you want to use to change FOV in JoyToKey.
Set up one for 'Mouse up' and one for 'Mouse down'.
Launch CoD and go to controls and assign each of the two buttons to their own instance of 'Hold to Adjust FOV'. Voilá.
Same procedure to set up an axis, but you will have to assign a key or button to hold down while moving the axis.