I found this app note from Microchip which may be of use to someone for battery operation.

Vdd Self Starting Circuit
Building on the previous topic, the same charge
pump can be used by the MCU to supply its
own Vdd. Before the switch is pressed, Vbat
has power and the Vdd points are connected
together but unpowered. When the button is
pressed, power is supplied to Vdd and the
MCUs CLKOUT (in external RC oscillator mode)
begins toggle. The voltage generated by the
charge pump turns on the FET allowing Vdd
to remain powered. To power down the MCU,
execute a Sleep instruction. This allows the
MCU to switch off its power source via software.
Advantages:
• PIC MCU leakage current nearly 0
• Low cost (uses n-channel FET)
• Reliable
• No additional I/O pins required
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