I don't know Cherokee electrics, but does it maybe offer a way to (very
slowly) charge the battery thru the external service plug?

My Cessna 172M has a funny diode-resistor circuit like that in the
battery circuit in which that's the only reason I can see for it. It
is not described in the POH or even the service manual. Otherwise with
a completely dead battery, if I used the ground service plug to start
it, there still wouldn't be anything in the battery to initialize the
alternator.

Flyback diodes don't usually have a resistor in series with them.