Well...I'll just stick with the BBJ and A319CJ! I am pretty sure there are no laws against operating them as a certified air carrier...just look at MGM Grand (even though they weren't TRUE Biz Jets...). The A319, I believe, is able to do this because it has minimal changes to the 'normal' A319 body and wing, and is thus certified for commercial use. The BBJ on the other hand with their body of the 737-700, and landing gear/wings of the -800, is not able to be used on commercial flights (unless they have received approval from the FAA...dunno, though). As for the seat-mile cost, I cannot help you out there...I have no clue about that. I can tell you (you probably already know, though) that if you charge enough per seat (and your pax are willing to pay it) that you will make a boatload of money off of those planes (isn't that a 'duh' answer!).
Hope I've helped...somewhat!