> How do PIC's cope with just a zener as the supply regulator  ... is
> that likely to compromise it's health or longevity?

The datasheet Vcc max for most PICs is 5.5V. Check it for the
one you eventually select. A zener supply would probably be stable
enough, depending on demand. Some small roughness in Vcc is
likely OK but you can, and should, add local filter caps if there aren't
any already

Something else to look at might be any power supply noise caused
by the FETs switching the loads, particularly on the ground rail. Often
you'll see this with inductive loads like motors, solenoids, relays etc.

Voltage spikes 0.6V below the micro's Vss can seriously damage
pin circuitry and even take out the whole chip if it latches. If you see
anything like that on the other boards you must protect the PIC's
inputs with small Schottky diodes (anode to Vss, cathode to pin).
Although it's cheap enough insurance to do it anyway with a few
little BATxx


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