Wouldn't work. Every decent cat collar is designed to come off with only a minimal amount of tugging[1], so even if you managed to get a collar on every feral cat in the city, they'd pretty quickly manage to get the collars off.

[1]: This is behavior that you absolutely want to have in a collar that's designed to go around the neck of an animal who's roaming unsupervised. Consider the (likely) scenario where the collar gets caught on something, and the animal panics. If the collar comes undone easily, the cat escapes from being trapped and saunters off, and you need to buy a new collar when you can't find the old one. If the collar is shut in such a way that the cat can't escape, the cat is likely to injure itself.

I believe maybe it would fall into the jurisdiction of local pet enforcement / RSPCA group to put bells on the cats.

But yeah, that would be no easy task. Trying to herd cats comes to mind :)