That's a very good question. Ideally, I'd love to find a system where everyone can just post photos, so they can draw on paper and then just take a picture with their phones or computers. Honestly, the best way might even be to run a game on a forum or messaging service, so you could just scroll through the replies. I think Skype chat would probably work.
Viewing post in Moss Creeps, Stone Crumbles
There are online drawing tools that would allow you to draw this in real time on a shared map - Drawpile comes to mind, and it IS possible to draw in Roll20 as well, although the drawing tools are very very basic.
If you want to keep a record of the stages of development, you could just take screenshots of the map regularly.