Adjusting FOV incrementally

[Bertram W. Wooster]

Is there any way to adjust the FOV incrementally using buttons/keys?

I know that there are key bindings for three preset FOVs (30°, 70° and 90°) and that there is another key binding to adjust the FOV incrementally using the mouse but these aren't what I'm looking for. I want to be able to bind two buttons on my joystick to increase and decrease the FOV in small incremental steps without having to take my hands off the controller to use the mouse.

Failing that, is there any way to modify the default FOV and/or the three FOV presets, say by editing a configuration file or something similar?