I don't see a problem vewnting out of a chimney. You just need a flu (it only opens up one way, and stops air if it tries to go the other way)to make sure no air/smoke from the chimney backs up into the attic when the AC is not running.
If u don't like that idea, vent out the roof. Cut a hole and insert a flashing, and a vent cap and viola. $35 bux with 25 feet of flex duct, sheetmetal screws and a tube of caulk for sealing included. U will need to get on top of the roof to put the cap on though. Keep that in mind... When u are done using the vent for growing u just stuff it with insulation. .