It is a slippery slope, IMO, but a contract is not valid unless one signs it personally. That has been the nature of contracts for a few hundred years. As a result, the Constitution was only ratified by a small group of people. At the time, many were unable to vote - blacks, women, men without property. The Constitution was not signed by everyone and therefore should not be legally binding with any respect to traditional common laws regarding contracts.
However, like I said, this argument is a slippery slope. Modern proponents of mob rule (democracy) would possibly try to use the argument to support their desire for majority tyranny - which was very far from Spooner's intentions. Spooner would have more likely condoned individual sovereignty and natural rights - aspiring to anarchy.
I haven't read every word written in this thread by Josf or yourself. So, please forgive me if I've missed something important. I would be happy to reply further if I have.