So the interior of the keel can't rust at all?
I don't really like aluminium, it's harder to work with and can have pretty serious problems with electrolysis...
I just send a e-mail to Robert H perry, but i'm not sure he designs steel sailboat. Anyways I asked him a few question and i'm waiting on a response.
Many commercial and custom yacht manufactures have a long history of fabricating with aluminum. Bob Perry has designed steel boats. A friend owned one, but much larger than you're thinking of having commissioned.