The current is the same anywhere in a single series circuit so you can add the diode either in the input to the 78M05 or in the GND pin as long as the load resistor is still connected directly across the 78M05.
The current would always be 75mA, regardless of the state of battery charge. For overnight charging as you requested, that should be OK but an ideal charging circuit would monitor the battery voltage, current and temperature. It would no longer be a "Extremely simple overnight charger" if all of those were monitored. The optimal way of charging is to monitor the temperature and note the increase as full charge is reached but adding a sensor and control circuit would make it quite complicated. Few Ge diodes can handle constant 75mA, the 1N34a is only rated at 50mA. Using two 1N4001 and one 1N5918 might work, they have a Vf of about 0.3V at 75mA.
It should be completely safe, even if the battery is removed. Bear in mind that if the battery is completely discharged it will short out the pre-amp supply anyway, not dangerous, it just won't work for a few minutes until the voltage has risen.
Brian.