Are they just trying to maintain memory in the ECM? A separate 12v system, regulated, with a motorcycle battery, or a battery like what is used in a home security system would solve that.
I would want the shortest possible lead between my battery and my cutoff switch, and the reason is that if the insulation is compromised between the battery terminal and the cutoff switch, the resulting short could dump a lot of amperage directly to ground, create sparks, and get hot enough to melt things.
I witnessed a buddy of mine place a hacksaw with no blade in it onto a battery, where it shorted out between the terminals, and welded itself together to the point that it was no longer useable.
My 2 cents - worth maybe half . . .