They really should make it so you can bind seat switching to anything. Like have a whole new command i.e. SwitchSeat 1, SwitchSeat2, SwitchSeat3. And as default your ini would look like:
1=SwitchWeapon 1 | SwitchSeat 1
2=SwitchWeapon 2 | SwitchSeat 2
3=SwitchWeapon 3 | SwitchSeat 3
That way, you could easily edit the ini and bind the switch seat commands to any of your weapon binds. For example, I would do:
NumPad0=SwitchWeapon 9 | SwitchSeat 1
NumPad1=SwitchWeapon 4 | SwitchSeat 2
NumPad4=SwitchWeapon 7 | SwitchSeat 3
I don't have assualt or shield gun bound because it takes no time to switch to xloc then mousewheel. And by using the arrow keys for movement, the stretch up to 1, 2, and 3 is quite a reach even if you did as TWD mentioned and used the 2nd column for you binds.