Just a few thoughts on this: about 25 yrs ago, I did a 'company' group of around 75 employees. I used my hasselblad (medium format) (film! LOL.. pre- PS days). Even then, when they blew up the print, the faces were (guessing now - because it's a distant memory).. but I'd say the faces ended up being less than 1/2" in a huge print. (maybe the print was 30" X 40" ??)
I'm just wondering how much detail you'll be able to get in 'iffy' light. (but I guess if you're shooting with a 12mp camera, it will help)
Anyway, I was lifted up high on some kind of crane with a platform on it. ... much higher than a ladder would reach.. but that's what it took to get faces that weren't hidden by the people in front of them.
I remember telling them that if they couldn't see the camera, the camera won't be able to see them. and to get all of their "blinking" done so they could attempt to keep their eyes open for the shots. .. I dunno.. saying goofy stuff makes them laugh to get their faces muscles to relax.
If at all possible, since YOU are the "director" of this shot, perhaps explain to 'the powers that be' (those who want the shot) that it would better serve THEM to maybe take a 30 minute break during the DAY to shoot this image. Try to find a sloping hill that they can be arranged on. (?). ... but assert yourself (nicely, of course) - asking for as much of their cooperation as possible. Bottom line is that they'll remember a *good* shot rather than a little bit of 'schedule shuffeling'. (hopefully that option is available)
I'm sure you're aware that daylight will give you the best crisp information in your image (I know you know that).. as opposed to the headaches of the flash falling off and not reaching those people in the back rows and/or on the sides of the group.
Anyway.. just an idea re: take the reins to control or change the conditions to meet their request .... because odds are, they are NOT aware of what it takes to achieve perfection with something like that.
... however, on the up side, sunset will continually be later each day for another 10 days. ... (days start getting shorter again on the 21st). .. so maybe you can find a place where the sun will still find them. (man o man ... talk about a 'light/time crunch!') Have you scouted your location options yet??