You can map that to the TIR Z axis (it's the axis that moves you backwards and forwards in the TIR software) in the game itself. Scroll to the right when you're assigning in the options menu and you'll see TIR, then simply give it's Z-axis the zoom function. I did however disable one half of the curve in the TIR software (not in game), because I have no need for zooming out from the neutral/centered position.
I find this works a lot more intuitive than assigning a snapview or slider to see the hud better when doing gunruns or bombing. Becomes very natural, also when viewing the mfcd's and cdu and other things.
Note that I do use the slider next to the APU you mentioned, to shift my viewpoint back and forward (since it has a nice middle position indent) and the TIR pause when needed, to do some stuff on the side panels (like adjusting radio's or lights, cdu programming, etc...)