You can connect the drain vent in the attic. The run should have slope so if any moisture gets in, say from rain, it can drain instead of pooling somewhere. You can buy round to oval duct pieces and run the stuff in the wall, but I'd be worried about the lower velocity maybe collecting lint in the bottom. Hope you are planning to exhaust it to the outside and not into the attic...a very bad idea.
While the drain line should be 2", you could probably get by with a 1.5" vent line (wait for the pros), if so, then you can run that through studs.