Something that spring to mind using a PIC.

Use a 12F629 or else with an external Crystal. Fed the CLKOUT signal to those OpAmp Filter.

The PIC will manage the Power-on/Off AND monitor the battery voltage. Could be interesting now... well just an idea.

One other using a 12F683.
This one Will produce a PWM signal on GP2 to generate a negative voltage for the OpAmp and do the same things as the previous suggestion.

Unfortunately those baby 10F can't run with an external OSC... that would be perfect here...