The official position of IDPA is that the match organizers should place padding where people are likely to kneel. This is not nearly as practical as letting people wear their own padding, but at least it has been addressed.
"Rulings" that are posted in the Tactical Journal are in themselves rules. While I don't care to lug around back-issues of the magazine in addition to my rule book, it is rather handy to have the updates/interpretations available quarterly.
Another thing to consider is that the relationship of the IDPA competitor to the SO is not the same as that of an IPSC shooter and an RO. The SO is more like a lifeguard to the RO's umpire. The SO should try to ensure that the shooter doesn't break any rules, rather than trying to catch the shooter in a violation. If I'm SOing a match, and I don't know the latest knee pad ruling, what's the harm? Even knowing that obvious kneepads are illegal, if a shooter with "bad knees" shows up to a match with pads, I'm going to let him/her wear them.