I don't think the cost to do it would provide much benefit. Its a great idea, but with proper employment of aircraft designs you shouldn't really need them. If no one is in the seats next to you, getting in and out isn't hard, and there isn't really that much getting up and moving around anyway. If you use a narrowbody like the 737 or A32S on a short flight, as they were meant for, you will have landed before anyone gets too restless. On widebodies like 767, and Anybus, no one has more than 1 seat to crawl over anyhow. Even thought it is inconvenient to squeeze past your neighbor, it might be even more inconvenient to ask him to unbuckle, move his things, and then stand up... only to do it all over again when you get back. Just don't rub your butt in my nose as you slide past and we'll all be ok.