It essentially boils down to running these 3 lines on a Pi that has Ethernet (running your new emonSD image)
sudo mv /home/pi/data/wpa_supplicant.conf /boot/wifi.conf
sudo ln -s /boot/wifi.conf /home/pi/data/wpa_supplicant.conf
Then the wifi settings are exposed on the /boot partition for editing on pretty much any PC in a file called wifi.conf. So if at anytime in the future you change your SSID or password (without editing the Pi Zero’s wifi settings first), you can just whip out the SDcard, edit it on a PC and refit, job done.
Yes it would be nice to see this as part of the standard emonSD image to make use with a Pi Zero or Pi A+ easier, perhaps @glyn.hudson will consider it.
[edit - also check out the Powering down a headless Pi with no network connection thread that was written as result of mentioning how to handle a change of wifi SSID or password above]