I don't think this will be quite as simple as you make it to be. You have kinda the right idea, it is possible to force flower outdoors, but if you are covering with plastic trash bags you have to keep a look out for mold. As for the light/dark period, 12/12 is really an indoor thing, since (in the norther hemisphere at least)you don't get 12/12 outdoors till late october/november(when its usually way too cold), you would probably be better off putting your bags on an hour or 2 before the sun sets EVERY day until the actual day light is to about 13 hours of light(near end of september) and then you wouldn't need the bags.