My point is that you are setting yourself up for a job that will end up needing to be done again in the future. Why bother when we can eliminate the task you asked about (measuring vDrop on the protection diode and changing formulas in the firmware)?
You didn’t start the conversation about a normal 105c, it was about Oshpark boards. If we modify the design to supply the divider with BAT+ instead of hooking the divider up to the protection diode then we’ll simply need to adjust resistor values to achieve the appropriate ratio for the divider. That’s a one time job, changing brands or models of resistor in the future will not have an effect on the voltage supplied to the ADC (unlike changing brand/model of diode). VREF will be constant, unaffected by the diode or absence thereof. Hopefully you see my point now?
EDIT: adding pic for clarity, see below.