Help driving 9V circuit with a 5V RTC clock interrupt pin (active low)
You could devise a scheme where you put the burden of bias for the switching circuit onto the 9V supply instead of having to introduce the coin cell. The circuit design idea below would support switching your 9V supply to a load. As shown the load could be up to ~400mA. The bias load on the 9V supply when the alarm output of the RTC is off is about 2mA with the shown component values.