Solved.
Documenting here to help others with same question.
In file: feeds/luci/modules/luci-mod-system/htdocs/luci-static/resources/view/system/system.js
Added:
if(uci.get('system','poe_passthrough')) {
o = s.taboption('general', form.Flag, 'value', _('Enable POE Passthrough<br>(24V from Main to Secondary)'));
o.ucisection = 'poe_passthrough';
}
This code not only displayed the new checkbox in System / System, but also updated /etc/config/system on [Save & Apply]. And yes: the PoE passthrough was controlled as expected.
It is working. Only I have one question. For my POE i have to set the GPIO to 0 to enable, and to 1 to disable. How can this be change in your code for LUCI?