I've now got enough extracurricular reading to last me a while. My friend Tom West (politics professor at the University of Dallas) just emailed me a dissertation by David Upham (a PhD student there) on the original meaning of the Privileges and Immunities Clause of Article IV of the Constitution. Upham summarizes his position by saying that the clause was meant to protect substantive national rights, as opposed to the typical interpretation by which it only prevents a state from discriminating in favor of its own residents. At 599 pages in length, it appears to be extremely thorough, and I look forward to reading it. But as I say, it should take a while.