I dare say, that yellow car is a little too messy of an install for me, leaves too many wires exposed. You can do the same kind of thing without so many exposed wires. I think it will be fine above the sub, but don't rest it on the sub. I would get 4 studs/special jam nuts or something and put them where needed through the back peice, then make 4 simple angle brackets bolted to the case. That way, if you need to remove it, you can just unbolt it from the floor.
If you have it above the sub, on mounts, attached to the carpet/fiberboard stuff that makes your firewall, it shouldn't have too many issues. A buddy had a computer under the seat, with the sub right behind it...it was in direct line with the vibration from the sub. I personally have not had a vibration issue, and I have motor mount insert which makes idle vibrate quite a bit....more transfer from the motor.
But why isn't inside the panel a good spot? That looks like it works great!