I noticed that /sensitivity directs to cl_sensitivity which is a client var. Hence a workaround I was using for a mod may be suitable for the engine.

It can be suitable for client use with a cl_zoomsensitivityfovthreshhold cvar making users able to set a threshold below which 'zooming' is considered to be the case.

So basically one can use
/zoomsensitivity to multiply sensitivity x float when zoomed
and
/cl_zoomsensitivityfovthreshhold to set the mod's normal fov

In R_SetupProjection(),

static float 	pfovX, norm_sens;

if (!pfovX) { //initially
	pfovX = dest->fovX;
	norm_sens = cl_sensitivity->value;
}

if (pfovX != dest->fovX) { // did it change zooming state?
	pfovX = dest->fovX; // save the current zooming state
	if (pfovX >= cl_zoomsensitivityfovthreshhold->integer) { 
		cl_sensitivity->value = norm_sens; //fov normal, stay normal sensitivity
	} else { //we're zoomed
		cl_sensitivity->value = norm_sens * cl_zoomsensitivity->value; // we zoomed, multiply it by zoomsensitivity
	}
}



I know know, it may considered workaround-ish but since sensitivity is a client var..