If anyone is still interested, I have just done this in a 2002 BMW X5.
It was quite simple - if you drop the centre arm rest and remove the leather trim, the "hole" is pre cut but filled in with a thin, metal blank. This is spot welded in around 9 or 10 places. These were a beggar to get out. After a couple of hours of hard work, I got the blank out. I am very pleased as it fits a snowboard through - easily! In fact, I could get a couple of snowboards (no bindings) and a couple of pairs of skis through. This still leaves space for 2 people in the back and a lot of boot space. Best of all - no luggage rack and a the gear is safe in the car. All I need to do now is smooth off the edges and put some edging trim round. I haven't bothered buying the official ski bag and plastic covers etc. as, when not transporting boards etc, the arm rest seals the space so no extra noise comes through. Very pleased with the result for a couple of hours work. I thought it may reduce rigidity of back seats - but as it is pre-cut, when removed, appears to be just as solid as it always was.
Feel free to pm for more info.