I could see the attic air situation working well in the summer but if it's the summer why not just hang a clothes line? I guess rain can get in the way. In the winter it would be less useful than using indoor air because the attic should be the same temperature as the outside. If it's not then you have air leaks and poor insulation up there and you have bigger things to worry about than dryer efficiency. Also attics are generally dusty. And you would need a really good flapper of some kind or else the stack pressure going out that pipe when not in use would be tremendous. Most vent flappers are designed for out going air.
Which brings me to another point. My favorite perk of using a dryer is that is removes dust from clothing. It certainly also removes valuable constituents of the clothing itself, but it does a great job of removing cellulose!
About the fan to boost dryer efficiency. I started a thread about this a while back.
http://ecorenovator.org/forum/applia...ryer-hose.html