Technically, according to USAWKS by-laws and the modifications stated within:
Rule 4-1-1 The legality of all uniforms will be up to the referee, and need not conform to the NFSHSA rule.
Thus if the referee is okay with it not being attached then it could be permitted. I would say however that as an organization it is important that everyone follow that same basic standards and we do try to follow NFHS rules as closely as possible, so I would tell your families that if they are in a situation where they need a hair covering then they should purchase one that now conforms to the NFHS rules regarding such hair coverings.