Assigning project specific messages to acs-kernel or acs-subsite seems a bit like a kludge to me. I'd rather create my own project-message-catalog package in that case. But then I'd loose the ability to distinguish between e.g. error.xxx and generic.xxx (don't know if that would be necessary though). Hmm.
Also we might want to enable content editors of a site to create their own groups, without having to create their own package. Not sure about this either. Some advice from somebody who used that in practice would be good ... Bruno, do you read this?