I don't think you'd have an issue with one of the leaders of the Rheinland corporations giving you an ooRP tech permission for a Rheinland transport considering the role play of the character. I know I wouldn't because you want it for the role play, not as some kind of in game power trader advantage.
It would make more sense to me to get an ooRP permission, as opposed to asking them if you can keep your transport your Rheinlander already owns. :PAnyways, good luck with it.
I think the main concern about something like that wouldn't so much be character preference, but company preference. Samura and Kishiro both have shipyards which build the civilian and military big ships in Kusari which are different from the civilian big ships built at the Oder and Alster shipyards. For repairs and maintanence of a Behemoth at Yokohama or Yukawa shipyards, specialty parts have to be ordered from Oder or Alster shipyard and then shipped to Yokohama or Yukawa to do the repairs and maintenance on your ship. So obviously it would be a lot easier for them to hire someone who will be payed to fly a company ship which is easy and cheap to maintain and repair. That is assuming that Rheinland and Kusari engines are different and we haven't adopted universal components (which I don't think we have, since Kusari engines are orange and Rheinland ones are green). I feel that when looking at corporations, a token of conformity comes from working for one of them. Mind, I also had a Rheinlander in Gateway Interstellar who the the Director of Trade Operations, but he flew a ship Gateway Interstellar would have in its fleet (Large Train).
Additionally, most shippers for the main shipping companies shouldn't really own their own ships. The company has a fleet of their own which is easy for them to maintain and keep up. They hire someone trained in flying Transports and that someone gets inside a stock Kishiro painted ship and fills orders for pay. If someone would own his own ship, he'd be more like a Freelance Trucker here in the U.S., where he owns a truck and a license to truck stuff and moves things for people who can't afford their own fleet of trucks for higher pay, but only because he has to maintain his own equipment.
But then again, this is a game and you're free to do whatever you want if you want to do it and think it'll be fun. That's just my take on it.
[8:32:45 PM] Dusty Lens: Oh no, let me get that. Hello? Oh it's my grandma. She says to be roleplay.
[12:49:19 AM] Elgatodiablo: You know its nice that you have all that proof and all, Bacon... but I just don't believe you.