Residual heat loss is most effective based on the fact that heat RISES. Your bathroom might gain heat from the hot water piping in the floor below, but the piping is benefiting very little from the heat above, especially given the insulation.
4" of extruded polystyrene attached to the bottom of the joists would give you R-20 between the garage and the bath. It is also possible to cut the insulation to fit between the joists under the piping, but it is much harder to get an airtight seal even when taping all the joints. Depending on the direction of the joists in relation to the piping (and your needed ceiling height), you might chose this method only in the area of the pipes. Individual joist spaces insulated from below can also be kept warmer by ducting air from the existing heating system.
Do your homework on the building code regarding the firewall between the garage ceiling and living spaces. Most areas require one, and some TWO layers of 5/8" drywall.