It would be difficult, but it's an interesting idea.
Among the sources of authority are common-law or non-statutory sources of information, including the common-law powers of the police to collect and share information for investigative purposes, and the defence prerogative under which the defence department collects information. These are non-statutory sources of authority that, by definition, exist outside of the statute that you're envisaging. It's not obvious how all of this would work, but it's an interesting idea.
What I would say that may be helpful is that in respect of the safeguards I'm suggesting for SCISA as a threshold of necessity and proportionality, and in respect of the requirement for agreements that create clearer rules, some accountability, and retention periods, there is no reason these safeguards could not be in a statute of general application that would apply to the sum total of the sources of authority. That might be one way to ensure that safeguards for the rights of Canadians apply regardless of whether SCISA , the Customs Act, some other piece of legislation, or the common law is used. That would be the most practical advice I could give you on that point, but it's an interesting suggestion.