I wouldn't approach this problem from a technical direction. If there is a browser based vendor agnostic opt-in popup for user tracking (not only cookies) you

I wouldn't approach this problem from a technical direction. If there is a browser based vendor agnostic opt-in popup for user tracking (not only cookies) you can outlaw and severely punish attempts to circumvent that. Given the time and resources courts really dislike the "welllll technically..." Argument.