No one is going to pay High Passage to travel on a free trader for the service anyway. The only way someone who is used to travelling by high passage would travel by free trader is if he has some ulterior motive. Perhaps the free trader is the only one going in the direction the passenger wants to go; perhaps he wants to avoid leaving behind records of his travel; perhaps theres some other reason. The bottom line is, though, that the presence or absence of the proper level of personal service isn't going to affect his decision to travel by that free trader one way or the other. (Especially since the food he'll get will be the same as that of the medium passengers).
So when Richie McRichguy shows up at the entry hatchway of the Slovenly Bastard, he'll either pay for a mid passage or he'll be told that the ship is full up already, in which case he'll whip out a High Passage (or Priority Passage Voucher as I call it) and get his passage anyway.
Hans
Maybe ships should have a Social Standing based on style, space, luxuries and how loaded with Stewards they are? High SOC characters risk societal demotion being seen entering the 'wrong ship' on 'civilized' worlds?