I don't think a preamble is going to save the day. To say you can randomly stop someone and randomly demand that they provide a sample of their breath or other bodily substance without reasonable suspicion or reasonable probable grounds, when there is legislation in place already under subsection 254(2) authorizing a police officer, if he reasonably suspects a person, is an exceedingly low threshold for investigative purposes in making that sort of determination.
You commented earlier that if it were administrative and not criminal, perhaps that would be all right, and you're probably right. The difficulty is, they don't know whether the person is going to blow 50 mg% or 80 mg% until they get them back to the station. So as a practical matter, the suggestion you made, I think, is unworkable.