Rather than toilets where cleaners might have to scrub them off promptly, I much prefer public sites, on streets. I think that they will get more attention
I totally agree with you that public sites and on streets are fab for attention. They will be absolutely necessary to keep the snowball rolling down the mountain, gathering mass as it goes. But attention in these times, while easy enough to grab, is really really hard to hang onto.
To hang on to attention and achieve a slow burn of steadily rising numbers of people who independently join in (with stickers, pink post it notes + pen, chalk, lipstick, any of the other clever and beautifully curly minded ideas that have been posted) will be fundamental.
Most of them will never have protested before. Never have participated in even tiny acts of civil disobedience. Will wildly over estimate both the risks of CCTV and the fall out from being caught. Many will see themselves as having a lot to lose, be it a job where perfectly clean noses are a must, or friends/family who can't see beyond a willy sticker and focus on that rather than the intent and aim of the message.
Those people could turn a small pocket of resistance into a diversely motivated army of thousands upon thousands.
IME toilets have long been disproportionately decorated for several reasons.
The privacy means even those less prone to making a point, or waves, feel the risk/benefit balance is strongly in their favour where prying eyes can't see and accuse, let alone prove an accusation.
I had friends at school, good girls, quiet girls, squashed girls, who endured years of vicious bullying. They took a stand in the one place they could do so in relative safety. It made them feel less silenced and powerless. It gave a massive boost of morale to see a bullying princess squawk helplessly, in front of her sycophantic court, as she tried to work out which of her victims had made a public, perfectly cutting, sore point prodding point at the Princess' expense. And I saw the less enthusiastic, self defence motivated allies snicker in hidden solidarity when they thought nobody was looking.
For decades and decades loo artists have factored in a major benefit of their chosen canvas. It might not have the foot traffic of the street. But neither does it have the street's distractions. It can offer the advantage of tiny moments of nothing to do ....but, squint, go "huh? ohhhh!". With some people walking out with a non rapidly fading memory, that can compete to some extent with the bombardment of clickbait, information and real life shit to do that descends as soon they have finished washing their hands.
The loo has long been where the first seeds of subversive resistance to one's personal oppressors are sown, with a temporarily captive audience to boot.