Someone with more experience may answer this but in my opinion the biggest thing you will need to worry about is wetness. Now that your plant has no leaves on it, it will not be transpiring any moisture, so the soil may very well stay wet enough to cause rot.
Not sure if you are closing the bag around the plant or just using it as a catch to hold the leaves (insulation I presume?), but I personally would not completely enclose it in a plastic bag.
You could probably just put the entire thing near a window, if there is one in your garage, and see how it does there, as long as the soil stays relatively dry, it will probably do just fine. And it would probably do fine in a dark space as well, but the soil, again, should probably be pretty dry.
I really have no idea if a plant that size would bare-root dormant store well or not.
I think most folks (I am not speaking from personal experience here) who do this store their plants in a dark and relatively dry space (slightly elevated humidity maybe but not wet) with cool but stable temps and reasonable airflow.
Another option for a plant that size is to just bring it into the house and keep it as a houseplant over the winter. A lot of people do that with plants of all sizes. You still have to be very careful not to over water them and you will likely need a mite control plan, but otherwise very doable.
Time to put the bananas to sleep for the Winter