> 4. By using very large value resistors (as I recommended), the current
draw,
> even if the pin is incorrectly set, will be negligible. A 100M resistor at
5
> volts is 50nA IF set incorrectly. I have seen power consumption increase
by
> 50mA when the pull-ups were removed from an unused port on an old Z80/PIO

This is a useless value for a pullup.
Electric field effects will dominate, and your "pulled up" input will be
bouncing all over the place.  Try something sane like 1-10k, which will
survive even near field E and H field events and usually remain in whatever
state you pulled it to.

Pullups on cmos inputs don't draw appreciable current.
NMOS Z80s are not CMOS, and they do draw current.

The safest, least cost method to secure unused pins is as outputs, low or
high.
Making them inputs adds ways for the code to have problems.
Shorting an output pin to the rails is not going to kill a chip. They will
simply output their max current into the load, and get slightly warm.

That being said, I would still put a resistor to ground on the schematic,
and not populate it, so that later, you can add things to those pins, using
the resistor pads.