So I found this site- Tzigla

Basically you pick a board, pick a square that's available, reserve it by clicking, download a template of that square, draw something within a time limit depending on the board (or you lose your reservation on that square) that fits the edges of whatever tiles are around it, upload it on the same page you downloaded it from, and then if it's approved it'll be added to the drawing.

Here's a finished collab example- quack, quack, pixel duck

Now the reason I'm posting about it here is that they apparently offer the possibility of integration with other art sites- link

I actually got to there from another site they've integrated with. I was just thinking it might be an interesting option for us to explore large scale collaboration art. Even just earlier today before I found that site I was thinking about that exact kind of thing because I'm currently working on a very large collaboration on yet another site. I'm not sure how well the integration would work out since all of the drawings we make here are on applets online and we don't have the option of using a template from our own computers or another site.