I have a 2012 super sport with a 140 sigma and the FIA master cut off switch, factory fitted. On mine when you flick the switch the immobilized light goes out so I guess everything is powered down. Having read the thread again, I am wondering why you don't just fit a simple switch, discarnect style, to the battery and then just switch it off when not using it. I realize you would need to lift the bonnet to do this but it's only 4 catches and when turning it off you would not do the catches back up until you turn it on again. As back up, a battery on the ground with a set of jump leads seems simplest solution. Or even one of those new lithium jump packs which I have seen but confess have no direct experience of.