There can be problems reserving your seat if you fly under a non KL code. From own experience:
- KL operated, UX marketed: could not select a comfort seat in advance, nor at online checkin. After showing my plat card, the purser allowed me to sit in front.
- KL operated, CI marketed: could not preselect a comfort seat in advance, even the platinum line could not do it. OLCI also didn't work. Managed to get a seat at airport checkin.
In both cases this was when the flight did not originate in AMS. If your flight originates in AMS, you usually checkin via the KLM systems, which give a better chance of success at OLCI, though advance seat selection will still be impossible - even via the KLM website.