I think you need a little backround on the prohibition of water ballast. Water ballast was outlawed because dingy sailors were carying water ballast on boats that required hiking like the fin and laser. A signifigant fraction of them incurred knee injury due to the extra leverage and strain. ISAF banned water ballast to stop this. Next people started wearing 3 or 4 sweatshirts over their other gear so they got wet and were ballast. Same problem. To counteract that, a maximum wet weight for all clothing was enacted.
When camelbacks were first becoming popular Isaf spesificly outlawed them except in windsurfing, because they are considered water ballast. In distance racing people have been looking the other way for years. I do believe NORs for distance race should alow camebacks, but should set a reasonable weight limit, maby 70 oz.

Now if you are wondering if a camelback could realy help out, on an A class cat, a 128 oz camelback on a 160 lb sailor increases righting moment by ~6%. Something to think about.

For buoys racing I have a hard time justifying why you need a camelback. On the otherhand, multihull sailors were not the ones the rule was enacted to protect

Eric