It sounds like what you want to do is very similar to what I do with my plants for dormancy. I keep mine on an unheated porch. I close the door to the house and open all of the windows on the porch. It gets enough heat leaking out from the house to protect the plants from freezing, but keeps it cool enough to provide dormancy.
Light is for sure the more dominant factor in providing dormancy. You need to make sure that you aren't keeping the light on longer than the sun is up. But it sounds like you have the shortening of the photoperiod worked out.