What would be appropriate to serve for a business lunch during Passover?

Depends on how strict you wish to be, but really anything you'd serve any other time of year but without foods that are not to be eaten during the Passover observance. But if you're looking for some menu thoughts, here are some I can think of:

"Caesar" type salad with grilled or poached salmon or chicken;

Lamb chops (grilled and chilled) with a taboule or other green salad;

Roulettes of chicken breast stuffed with herbed matza meal;

A diet plate of cottage cheese & fresh fruit (just don't serve this to anyone having any of the other dishes!!!);

Steak with a side of mushrooms & greenbeans.