From the pictures, it looks pretty straightforward. Seems to be a decent size space if one rips out the original sub, and it should be possible to find a box that fits. Difficult to judge the size from the pictures, but there seems to be enough space for a 15-20 litre box, which is enough for a decent sub that gives a hell of a lot more "ompf". One should be able to source the signal without cutting or removing anything of the original wires. I'm also in contact with a company that makes custom made upgrades to virtually every car. They said they haven't had much demand for the MGS6 yet, but it's on their list. But as I said, they already have kits and wire harnesses for the MGS5, and this looks very similar.
I'm also thinking there is yet another option, and that is to actually add a strip of styrofoam or similar around the edge of the entire boot, and raise the floor slightly. This would give the option to add a slim but wide box, pushed right up against the seats, like the custom solution for the mgs5. This would also leave some storage space under the boot lid.
This is a bass upgrade for the MGS5.
Should be able to do something similar or smaller for the MGS6.