Yes, it's possible to make such a system. However, limiting the requirement to such a system is more difficult, especially with the commercial options out there already that are based around an ID or other potentially sensitive info (e.g., credit history, mortgage records, etc.).

It's not what the OP is about, but check the requirements for various states in the US linky, and you'll see how many of them have identifying requirements built right into the law.

It's easier to oppose the concept generally than to argue specifics, especially if you're among those that believe identification is part of the purpose of these laws not an avoidable side effect.

I've always thought that requiring the sites in question to have easily identifiable metadata to assist with client-side blocking made more sense. Parents can block it on their kids' devices, and adult pervs can remain anonymous.