So it doesn't get lost on another thread, I'd been wondering if there would be any impact of the lack of this clause in the Constitution? It wasn't really enforced consistently till later, but there could be some ipact - and it would be easy to have a different delegate to the Convention who doesn't prpose it or just have it kept out when taken out the first time, instead of quietly put back in.
It would be an interesting thing to have the Northern states citing States' Rights. Of course, there would be the argument of states needing to give Full Faith and Credit to others' laws, but a good counter would be that this isn't an issue of the slave states' laws, but rather of whether or not people qualify as human beings. (WHich sounds really obvious to us, of course.)
This might be one way to end slavery more rapidly, since states would know their slaves might not be returned - especially in the Upper South. Perhaps it would be even more expensive to try and keep them and it would get the Upper South to start gradual emancipation earlier.