Carroll.

I was faced with this problem several years ago. At that time, I found a solution but this implies a little extra hardware around the controller.
You must use a STATIC CMOS device ( this is no longer a constraint I suppose).
Use an external clock oscillator together with a gate to the clock input of the processor.
Use this gate to stop the clock and also the processor as long as you need.
I remember the clock gate I used was formed by two D-type bistables in order to avoid spikes and to have always the right phase.

Regards,
Cristian