It's true that the Mark V uses all 8 switches. You can definitely use the 4x1/4 cable to switch selected functions through the external switching jacks.
Another thing to consider is that you might still be able to use the 1/4" jacks. For example, if you were to plug an amp into the Switch 7/8 jack, it would work fine. The side effect would be that you'd switch tuner mute and reverb on the Mark V when you switched the functions on the other amp. But, if you're only using one amp at a time, it doesn't matter if the reverb or mute is on when the Mark V is not active.