I considerably reworked the design, and now have one of those boards built up.. I put a P channel MOSFET on the power line going into the MCP73831 LiPoly charger chip, which is the Pi 5 volt line. The power line out, has a Schottky diode, and then goes to the Pi 5 volt line. I use a comparitor across that diode to switch the input MOSFET on and off.
If I’m running from the Pi, and charging the battery, the cathode of the diode has 5 volts, while the anode has the about 4 volts coming out of the charger chip, keeping the comparitor off. When I disconnect the Pi power supply, The anode has the 4 volts from the battery, and the Pi 5 volt line has that, less the voltage drop from the diode. This sets the comparitor, which turns off the MOSFET. That way, I won’t be feeding a low voltage back into the charging chip when running from battery.
After the Schottky diode drop, I get about 3.7 volts on the Pi 5 volt line. That’s enough for the 3.3 volt LDO regulator on the Pi. Most 5 volt peripherals won’t be able to run, but the Pi and 3 volt peripherals will be fine.
This is a bit of a late entry on Pi day, but rounded, today is 3.14.16