Is your soil frozen? If not, go ahead and plant in the ground. While dormant, deciduous shrubs and trees are very adaptable to/tolerant of transplanting and the plant(s) would be much happier in the ground with the roots protected from cold and able to access moisture than it would be stored as-is in your garage.

If the soil is frozen....and that's not often a concern in zone 7.....then go ahead an pot up the shrub using your plastic bag method or a large nursery container. Check periodically to make sure the soil surrounding the roots is just barely moist to the touch -- you may have to water occasionally. And store in protected location where the temperature is not going to fall much below freezing. Roots are the most vulnerable parts of a plant to cold and can suffer irreparable damage if exposed to severe cold without the benefitting insulation of the ground.