>>Most PICs are rated to being able to source up to 20mA
>>and to sink up to 25mA at any pin.
>
> My understanding of the data sheets is that this would apply to any pin that i
s
> not a supply pin, i.e. this is the maximum current you could put through the
> protection diode.

No, the protection diode doesn't enter into that.  The protection diode is
to protect against externally applied voltages above Vdd or below Vss.

The output sink and source limits are the maximum currents through the
output driver FETs.  If you exceed those, the voltage outputs will not
be met, and the part may malfunction.

Note that in addition to the per pin source and sink limits, there are also
per-port limits.  You can't source or sink 20 mA out of ALL of the pins.