You might want to consider just draining all of the water. if the pipes are sloped properly, this may not be hard to do. Some RV antifreeze in the traps should keep the drain lines from freezing (toilet, sinks, etc.).
Moving water tends not to freeze, but just sitting there, if there are any air leaks, the pipes could freeze, insulation or not. If the pipes are up in a joist bay, rather than crossing below, if you put some insulation beneath them, the heat from the house would be trapped. You need to be careful about moisture barriers so you don't end up with condensation or rot.
So, it sort of depends. Maine can get REALLY cold, so you need to take this seriously. Maybe a picture to show how things are arranged under there, or a diagram might help provide better responses.