I am highly skeptical everyone will play by the rules, even if you could get people to agree on paper.
The difference between something like "unlicensed AI training" and, e.g, Nuclear weapon non-proliferation is that nukes are realistically only a deterrence against invasion. There are no economic benefits to having them in a stockpile. They're hard to make, and the manufacturing and testing steps are pretty obvious to an observer.
This is opposed to AI training in a data center that might host remote-stream video games, hospital infrastructure, protein folding, etc. How will we be able to police that in any realistic fashion? Will China or the US allow unfettered visibility into all data centers data streams to outsiders?
And again, the incentives: Imagine having Astra v3 or Opus 8 while everyone else is stuck with a lobotomized version of GPT 4o.
What about the military applications? The weapon of the future is autonomous drone swarms. If I'm a superpower, I'm pouring hundreds of billions into that.
About the only realistic way this is going to work is if we are able to solve alignment in a way that doesn't significantly hinder AI development. If it imposes even, say, a 20% handicap, people will ignore it.