Air Wick, Febreze, Glade and others manufacture bathroom air fresheners that after a given time release a spray mist. Not my department but the wife just orders cannisters using Amazon. Fully automated and programmable. Going to Amazon and searching Air Fershner Spray Automatic will give you plenty of results. Just find one and hack it to see exactly how it works. Then figure out a cannister and how to charge a cannister. This is where I would start since it's already been done.
Ron