1. first, double check to make sure there aint a drain hose. if not, this idea could work. the hole would have to be at the lowest point on the case. but even then it may not drain all the coolant out, and youd have to leave it open til the left overs evaporated. while its open hot air would come out the hole, which would reduce the efficiency of the heater and defroster. then, youd have to be sure to seal it up real good, to prevent hot air from escaping.
before making a hole, u may wanna see if u can loosen the screws on the bottom of the case and seperate the halfs a little. if theres any coolant in there it should come out.
2. the core would have to come out for this. without adapters and a way to pressurize it, a workaround would be to empty as much coolant as u can from the core, then dunk it in a bucket of water, with the tubes pointed up and the ends of the tubes above the water surface. then check carefully for realy tiny air bubbles coming up from the core. if u see this, that points to a leak. if u can blow some air into one of the tubes, that could make it easyer to see any leaks. btw this allso works for the a/c evaporator core.
if it is collecting in the case, chances are it would find its way out thru any small leaks in the seal between the case halfs, or where the tubes go thru the firewall, or any other place where there aint a water tight seal. if it did build up in the case, airflow thru the core would be reduced or elliminated, and the heater and defroster would not blow air, or blow reduced air, depending on how high the coolant level was in the case.