alt Hacker News> advertisers will just require that you compile their library into the first party js code, negating any benefit from such a security model.
It will become harder for advertisers to deny responsibility for ads that violate their stated policies if they have to submit the ads ahead of time. Also site operators will need a certain level of technical competence to do this.
More likely, advertisers will need you to insert a "bootloader" that fetches their code and passes it to eval().
Alternatively, they might require you to set up a subdomain with a cname alias pointing to them (or a common CDN), negating any security benefits of such a practice.