Then one day someone cuts up that decking with a sawzall and gets an unexpected surprise.

That's possible, but you'd need to clearly mark it.

For two feet it's overkill, but normally, especially if there is the possibility of going between the two structures, you'd dig down 4', lay down your cable so that it is going down vertically in both structures and is well below the frost line where you bury it horizontally.

Yes, true. That cable has a special marking and there are even kinds that are solid enough to be driven over with tired vehicles. But those are super expensive, far more expensive than the kind that can be buried, and they are serious work to strip and connect. In a lot of cable that I bought there was one chunk of that stuff, about 2" thick for what looked to be tri phase, 40A+ground (so five wires + the steel cover). Bending radius about a mile :)

I'm sure there are thinner more pliable versions of that stuff available as well and I sometimes see the equivalent tie wrapped for temporary industrial applications to cranes and bridges.