im sure there is a better way of doing this but what comes to mind is. the on button triggers a 555 which has a frequency of something like .5hz then the 555 output turns on a transistor which turns on a relay and the realy self latches, the relay supplyies your circuit board. that would work for a toggle style swich not a momentary, youd need to figure something else out to break the latch when the mom switch is pressed again.
there is a bounce when the relay turns on, dont know if that would matter?