Can the spark core monitor it’s own VCC? As a battery rundown mechanism?
Nope. The ADC positive reference is the 3.3V analog rail.
In theory you might get something by monitoring Vin via a resistor divider and use the output of the LDO as a 3.3V reference, but the results will be crap.
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Thanks @AndyW! I was hoping there would be some trick like:
https://code.google.com/p/tinkerit/wiki/SecretVoltmeter
Yes - that’s possible because some AVRs have internal references and divider networks that you can use to accomplish this. The STM32 in the 
doesn’t. Sorry.