Here is a listing of kosher restaurants in the Boston area. Mostly casual (falalfel and delis), but Taam China might be nice enough.......
I have heard that some of the fancier hotels will bring in a kosher meal for you (catered, perhaps?), so maybe setting something fancy up in a private room at the Four Seasons is an option.
re: Science Chick
Agreed, Taam China is surprisingly good for a kosher Chinese, and is the only one that's even remotely fancy. They have a full bar as well. You might want make a reservation since your party is larger than a four-top.
Taam China Restaurant
423 Harvard St, Brookline, MA 02446
A sushi restaurant would not be considered an option for anyone who kept strictly kosher. Sure, there are non-treif items to eat, but the restaurant has alot of shellfish around, which is strictly forbidden. Any keeping strictly kosher would not eat anywhere where the kitchen was not certified kosher, regardless of what they serve. I have friends who are Chassidic, and it is the process of koshering that is important, as well as the types of foods prepared and the methods used.
Unfortunately, even though there is a huge Kosher community, no one has bothered to open up a fancy kosher restaurant. I wish they would, many of the business guys in my office keep kosher and we always have to cater in for dinners.
Depending on just how strict your guests are, you may want to do a catered in dinner. You can set a nice table with plastics... a little cheesy but it may be nice. I believe the Butcherie may also have prepared foods. Our office uses Milk Street Cafe and Catering by Andrew!
Like others said - there aren't any real formal kosher restauraunts in Brookline other than the chineese place on harvard st. Taam China II in Newton is ok, strictly kosher, but again its chinese... It is a popular place for kosher business meetings! The food is bland and greasy, but kosher. IMO, Taam China in Newton its more comfortable than the place in Brookline, - it feels like a real restaurant.
Rubins, which is strictly kosher is very causual, fleshig, deli style - I'm a veg, so i can't vouch for the food. My carnivorous colleagues like it.
If you call ahead - Andrews might be able to prepare a meal for you, as take out.
Ruths Kitchen is supervised kosher - but its take-out chineese too.. i don't like it.
Cafe Eliat on harvard is a dairy pizza / pasta place -- not very good either
Rami's on Harvard - has a hechsher, but is a VERY casual felafel joint..
Sorry, but the options for kosher resto's in Boston are VERY limited.