Cool, yes. An interesting engineering exercise for sure. But I don't understand to what end? Even an old, not-very-limber, fat guy like me can still get on and off a solo seat with a tour pak in place. So moving it back doesn't prove to be an aid I understand. But there has to be a lot of mechanism there. And anything that can go wrong eventually will. If it fails in the rearward position you're suddenly lacking any back support at all.
Simple square post like the Corbin backrest gets the job done. You can do that in front of a tour pak that's mouned in a forward position but has no pad on it. Or... well, there are other options that are far less complicated.
It's cool. If it's a show bike or one just like to go after something like this as a project just because you can; more power to you. But as an actual solution to something it seems a solution to a problem that doesn't really exist.