i'm not exactly sure about the softmod either.
i dont't think you can.
i do know that for the v1.0 you can flash half of the bios with custom and leave the other stock. you would just need a switch to use the different bioses.
as far as hard mods, you can definitely turn them on/off.
duox2 chips are cheap and reliable. you would just need to turn on the xbox with either the power button or eject button to turn it on/off