Considering that you can scan the clock in a few microseconds, the arduino should be asleep for more than 99.9% of the time if you're waking it at 1Hz from the RTC. So that means you're getting the vast majority of the power savings available.
You could probably slow the clock further using an external counter but my intuition is that the external counter will use as much or more power as you would save in the arduino.
A crazy thought I had is that (I think) any interrupt can be used to wake it. So you could theoretically connect your RTC 1Hz pulse to the T1 input pin and use the timer1 output-compare interrupt to wake the arduino after a specified number of seconds. Have a read of this and the datasheet I guess.