Hi Martin - some good ideas so far.

For shelter-in-place plans, I think you'll want to add to your flashlights some battery-powered lanterns, whether fluorescent or LED. Personally, I like LED lanterns - many have variable output levels and can really extend battery life.

I like this style lantern:
Essential Gear lantern - 12 LEDs, runs on 4 D batteries, adjustable output, claims 40 hours output at max level. You'd need multiples of these for 20 people - output would be enough to read off a desktop, sufficient to get around a bedroom room if hanging high in a room...nightlight if put at lowest levels.

This one:
Innovage 12-LED lantern is very similar in output, mechanism, etc., although not as sturdy as the first one - but I think it's definitely adequate for a shelter-in-place scenario, and you can't argue with the price. They had them in the Boscov's brick-and-mortar stores I went into about a year and a half ago.

I've heard good reports about these lights on CPF:
Energizer Folding Lanterns - More compact, fluorescent, two light output levels and the newer versions have a single amber LED for very low output.

For 20 or so people stuck in your office, I think you'll probably need to think about some cards, board games, activities to keep bored, uncomfortable and nervous people occupied. Perhaps not office Twister, but something along those lines.

For other instances, where you need to get out of town - emergency or non-emergency - you should look into as many alternate modes of transportation as you can. During the '03 Blackout, when I was still living in Brooklyn, I had to get back to work the next day (car service) and home from work without subway service. So, I got a crash course in the bus system, which I'd hardly ever ridden in my 11 years in New York. I'd recommend you get a stack of all the subway/bus/LIRR/Metro North/Amtrak maps and schedules you can, having them in the office and ready for a day you may need them unexpectedly. Also, you'll be right by PATH train and the ferry service - you might want to get an idea of how to cobble together transportation, from getting off Manhattan and then get moving from there on (I could get transport across to New Jersey, but once there I would have no clue of what transportation's available).

On the blackout day, one of the best things I had with me was a small AM/FM radio and earbuds. You could easily get a stack of these for your employees on your lunch break from the on-street vendors right around the WTC area (I remember the "Coby" brand being about $5 each - you could probably haggle that down if buying 20). I heard the earliest reports of the scope of the blackout and the realization that it wasn't going to be fixed that day helped me figure out what to do - I was stuck at an above-ground subway station, so lots of people were just hanging out on the platform in the sun (beautiful day), and weren't even thinking of going downstairs (dark building) to get onto the street. I had to talk several people into believing me that there wasn't going to be a next train that day.

Also, I couldn't get cell phone service to get anyone within the city, but I could call out of state (got ahold of my parents) and upstate - got my father-in-law in Buffalo. I'm sure you already have a contact list for yourself - perhaps you could get your employees to make up a list for themselves to have at the office. On 9/11, I was uptown and miles away from the problem, but I was fielding calls from family and, eventually, acquaintances I hadn't heard from for years, convinced I was in the middle of it. A couple calls out of state to friends or family who can reach the nearer relatives with the news that you're OK can make a big difference to your family.

You might want to see if you can get a corporate account with one of the many car services, perhaps one that has some of those extended van/small buses in their fleet - first to contact the car service in a non-emergency disruption, like another blackout, might be able to book enough transport to get all your employees off the island before the real crush of departures begins.

Another thing - a stash of office petty cash to handle such things as transportation, whatever else might come up.

Good luck with the plans - I'd love to hear an update as you build up your supplies
Dave