Yep, if it is awkward to 'paint the map' (perhaps an astrolabe/spinny-clock-with-sun-and-moon[1]-ornament side-graphic could do all the work) then just vary the other clues. Not just the freeze/thaw cycle, that is, plus the mega-fauna changes where compatible; along with the caravan/etc cycles, also have the typical visitations of fauna give the clues. (Because days whiz by, compared to movement of creatures, nocturnal/diurnal tendencies are going to be awkward, except by representing longer night-times/day-times with a shift in distributions of numbers, where seasonality isn't the
main (dis}qualifier.)
And some of that will be dependant upon seasonal migrations. Depending upon the 'latitude' (even hemisphere, or equivalent), you may get any given creature as a winter
or a summer visitor only (or Months 11-3 / Months 5-9, whichever range matches your spring+summer - if the worldgen now allows for this). Or if
'all myths are made true' (potentially) in this procgenned fantasy world, just allow/create some of the more fantastic life-cycles where seasonality is represented (perhaps mostly hidden, until/unless capture and even trainability makes it an in-fort occurance) as an annual-cycle were-like (one-way?) transformation.
Sorry, again. Getting far from the OP suggestion. (The astrolabe HUD-decoration/whatever seems an easier indicator, especially while overseer-infravision is assumed so long as there are overseen-eyes have already seen any not automatically revealed spot.)
I could actually see Steam-era graphics doing it, and I suppose I was already convinced that they
could do the ground-slope shading/tinting in continually changing manner to represent the changing light-source angle as each day (and night, assuming a fiction of an opposing-moon casting its colder silvery light in exact counterpart) if it wouldn't induce epileptic reactions by its free-running frequency.
Seasonally, perhaps this could be reduced (assuming the terrain graphics
could be made sensitive to and representative of an illumination-angle, which they probably can't if they're the fixed assets I actually assume they are) to merely 'wobble' to represent the date-appropriate mid-morning solar position (as if casting longer, lower, redder shadows in the depths of the year; shorter, higher, yellower illumination at the heights of summer).
Though this would surely produce calls for more realistic 'tropic' behaviour (light more from NE rather than SE, say, for half the otherwise two-seasonal (dry and monsoon periods, if lucky/unlucky) yearly cycle).
And anywhere deemed as underground really should not get
any graphical sop to whatever it is the game interface thinks the Sun might be doing aboveground, either by daily or annual measure. Except maybe for
very transient occurances, once or twice a year.
[1] Phased, even if we're going for the fiction (at least in our world) that the Moon only ever rises as the sun sets, and sets whenever the sun is rising again. I've known adults who refuse to acknowledge that the
Death Star Moon is visible
right now in that day's sky, and adamantly won't look up to clearly see it.