what you mean run it up under the seats like a honda?
Not seen it done but probably because it would require ninja (TIG) welding skills to do nice joins on such visible (and thin) stainless pipe...that's a lot of very custom sections/bends of pipe and expensive welding.
You could easily spend £500 plus on something like that. If you seriously want something liek that made hmu I wouldn't be able to use quite as thing pipes as most aftermarket pipes have (they're like tinfoil, very hard to weld) but if you took the front section from a normal aftermarket kit and then linked off that to do the rear section it might well be possible.
Probably still a terrible idea, but possible. Could have one pipe each side if you wanted.. go nuts.