I don't think any part would make a national SLI illegal. You could not however set up union standards to be admitted to that list. I don't think you will ever see a national SNL for many reasons. The biggest being what airline CEO is going to hire pilots off the list to place ahead of his employees and pay them higher wages. Even with a national SLI the airlines will higher pilots off the street and not on the list who will go on the bottom. Cheaper labor and you keep your pilots far happier with new hires coming into the bottom of the list instead of the top.

Most airline managements would simply not join into any national seniority list. Keep in mind that seniority lists are controlled by the airlines not the pilots union. Union control over a seniority list only exists to the extent that they negotiate in a contract. As a example DALPA the pilots union at Delta could never have placed Comair or ASA pilots on the Delta seniority list even if they felt it was a brilliant move. Delta management controls the seniority list. They would have had to agree with DALPA that they wanted the list merged. They didn't and would never have merged the lists.