Even supposing we could somehow get the political will to do this, how would you write such a law? What counts as “AI frontier research”? How would you write a regulation around that that isn’t trivial to bypass without banning general computing itself?

As I said in a sibling comment, we're fortunate that training modern AIs requires large quantities of specialized compute. We just have to restrict GPU sales and outlaw GPU farms. I don't deny that it would be a seismic, controversial change, but I don't think it's terribly hard to implement if we can reach a consensus that we want to implement it.