I have written a simple article which might be of use to some people, measuring the PIC Vdd (supply voltage) using no external components.
This builds on an older Microchip application note designed for outdated parts, so it isn't a new idea. However it works well enough for simple battery monitoring and can be added to any project on a supported PIC.
That takes me back to the real early days when we used to measure battery voltage with one resistor and cap on a PIC TTL digital input pin. As the cap charges via the resistor, the TTL input switches LO to HI at a fixed voltage (independent on Vdd) so like your Fixed Voltage Reference module it can measure the battery voltage in comparison.