All the aircraft have at least a curtain, but it is rarely used. I believe the Falcon has at least one set of galley doors that close it all off, but I have no idea about the NJI Gulfstreams or the BBJs. Mostly you have good conversation to fill the time. You're right about only company or flight related materials allowed in flight and the customers could be looking over your shoulder at just about any time- especially on the smaller aircraft where there is no flight attendant and the food and drinks are right behind the cockpit.