For any future flyers: the AF Facebook team was able to select my preferred seat at -48hrs, at that time the seat selection becomes available to AF on this particular codeshare. For the return on KLM nothing was possible before OLCI at -30hrs, however I was lucky enought to have 1A still available (could this be due to my FB Gold status?).
Other than the seat selection no problems whatsoever and perfect service on both AF and KL long-haul flights!