8L TDi's are not that popular on this forum to be honest dude... 1.8t's are generally what most people own here.... personally never done a catch can on a TDi so could not comment but in principal I would imagine its the same idea as on most engines where you are re-routing the PCV system to a catch can before it goes back into the inlet tract...
You may have more luck finding info out on a forum that has Mk4 Golf's... they tended to be a bit more popular as a TDi than the 8L A3
<tuffty/>