So one might hazard a guess that there is a hard limit somewhere in/below the low 50s, Fahrenheit, where even the staunchest CMOs will be unable to perform. Then somewhere around 54-55 F, the CMOs can open (they may or may not open all the way reflexed (if that is their normal habit), but they can at least open flat), whereas other daylilies may be sulking and struggling.
However, having watched a few plants early this season, I am coming to the conclusion that it is perhaps not so much about *only* "cool mornings" or "cool nights", but daytime temperatures in the preceding day(s). And the more cool days (in a row) there are, the harder it is for even the supposed/suspected CMOs to open fully. I am not yet fully convinced, but I am leaning in that "cool preceding day(s)" direction.
Another factor is the way a cold morning is arrived at. There is the low-50's morning preceded by a fairly comfortable evening, perhaps with temperatures remaining above 70° until midnight. These aren't too bad at all. Then there is the coastal cool morning, preceded by a blustery afternoon, with heavy fog blowing in at dusk which drops the temps to 55° shortly after sunset. This greatly slows bud development—which is at its most crucial stage the night before opening—resulting in smaller flowers with semi-turgid segments which not only do not open fully, but melt and fade under clear skies. Patterns? Extravagant edges? Rare, under those conditions.
However, as you mention, after a run of hot weather, that fog bank can be moist and refreshing, and the humidity boost makes for a great daylily show. Some of the best blooms I've ever seen were on mornings with heavy dew, or overnight "marine influence" (a new-age, left-coast term for mist). For example, see my image of Heavenly Dragon Fire
in the database, which was taken around 10:30 on an overcast morning following one of those rare humid nights. It's always a nice flower, but under my typical conditions it doesn't recurve this much, nor does it attain this kind of color saturation, and can be dulled considerably by noon on most days.
I find myself gravitating toward daylilies which recurve, simply because I know I will see a much higher proportion of presentable flowers. Also, breeding with CMOs, EMOs, and "quasi-nocturnals" will help ensure a decent percentage of seedlings with good performance in my climate. I also look for late-blooming daylilies, because they'll have optimum conditions for their first bloom. I think that the temperatures throughout scape development also play a large part in bloom quality, which is another reason why our long, drawn-out spring weather isn't the best for early-season bloomers.
As you say, a run of cool days is difficult for almost any flower to overcome. Best case scenario, there will be plenty of stiff, pointy, green-tipped sepals to look at. It also seems that daylilies prefer narrow day/night temperature swings when blooming. If cool weather moves in with a series of 55° nights and 70° days, the daylilies will seemingly adapt to that rhythm, and bloom 'acceptably', whereas when nights are 55° with 85° days, they suffer. I imagine most of the glamorous catalog shots are taken under conditions of high humidity, 70°+ nights, and 80-85° days, whether in a greenhouse or not. Of course, the greenhouse prevents sun damage, so the pictures can be taken later in the day, if necessary, in order to maximize development of ruffling, teeth, patterns and other features.