Ha, if market research was that simple.
Responses (How many orthodox Jews were on the flight to/from KBP/JFK) so far:
"Most of the flight"
"a few"
"half I guess"
"a handful"
You would have to sell to the general public as well if you want to have a chance of making money.
You have to ask yourself: Frum Jews aside, is this a good business? Do the numbers add up? How many sandwiches would you have to sell to cover all the costs? What else can you sell alongside the sandwiches to generate more revenues? Salads, Drinks, Coffee?
Maybe you should advertise as a Kosher/Halal stand. That way you might get the Muslim business as well. Or maybe Chabad would be interested in doing something with you and you can use some Yeshiva Bucrim on Shlichus to work for free.