Frank Ettin, If you’ll notice, the statement you quoted was not an appeal to the Constitution if you allow it to be qualified by statements which you did not quote. It was a mention that the government of the US was forbidden by the Constitution to infringe upon the preexistent, inalienable, and natural rights of man. The intention was to momentarily sweep away the relevance of the law concerning the topic (discard the law as a red herring to focus on the moral aspects).
The Constitution is, as you stated, a red herring from the standpoint of one man violating another’s Constitutional rights because it regulates the government rather than the people.
However, mentioning the Constitution was to point out that it recognized the rights of the people. The Constitution binds the government to act towards the people in the same way each of the people is bound by Natural Law to act towards each other. This suggests that the laws of this nation should mirror and reinforce a system of morality based upon the concept of individual liberty.
IMHO, The founders drew upon sources far beyond English common law. I think it can be shown they drew heavily from their own sense of morality, which in turn was shaped by early enlightenment thinking that treated morality as a science to be examined through logic and confirmed by investigation of real examples. They examined legal theory and examples of applied law (with English common law by far the most prevalent) through the lens of the moral sciences. I firmly believe their goal was to create a government that would act as a moral entity would under Natural Law. I would further suggest that their view of Natural Law, as applied to morality, was stated concisely by John Locke : "Reason, which is that Law, teaches all Mankind, who would but consult it, that being all equal and independent, no one ought to harm another in his Life, Health, Liberty, or Possessions."
Keep smiling ... it'll just make 'em wonder what you're up to...