Ah, you got there before me with the "Trade Depot-like" suggestion.
I was about to follow up my own post (but, yeah, had still forgotten about hidden deaths, in the latest version) to say that you'd need to have to introduce into the game some form of "make light-source" and place such items in strategic locations and then maybe (under the current engine limitations) switch mode to see what areas were dark or not. Which you just covered (at least the switch mode bit).
Beware, of course, that we're not going into "copy Minecraft right back again" territory. It needs to be as unique as it can be made, and not just "put torches on the walls and we're happy" time. Certainly, if we ever get so far as to have random monsters spawn in unlit+unobserved areas, it'd be a pain to look original. Even though MC isn't the originator of that style of mob generation, either.
Personally I'd suggest "oil tracks" one could lay down, to create a linked set of illuminated corridor lighting along the ceiling, walls and/or floor. Needs to be kept supplied by an oil production or similar (thus not ubiquitous, and needs supporting). Gas is also a possibility, but for some reason I see this 'artificially poked' 1400s-type tech level as being oil-fired, and we already have an oil-producing capability (a vegetable oil, via presses, not mineral oil which would bring its own challenges and perhaps MC/Tekkit-looking similarities) which could be subverted/advanced for this purpose.
Apart from various essentially perpetual sources (glowing fungii extracts, for example although that'd not be strictly everlasting either, while certain ores could be considered useful and essentially be light sources forever), I'm not entirely sure I want a "Lamplighter" job, either to maintain all underground lighting systems (topping up, if not tube-fed or otherwise automated) or to activate all overground lights during the 'night' (not currently a concept that exists in Fortress Mode, due to the seamless passage of day/night, at least compared with the various inexorable calendar progressions), but that's not a great dislike and I could see it being implemented at some point where lighting actually becomes of concern.