I think for this to succeed you'll need to organise it along the same lines as the aforementioned Exquisite Corpse exercises, i.e.:
- Decide on a unifying "theme" for your mural
- Decide on how many "tiles" it will consist of
- Select a volunteer to be assigned to each "tile"
- Select a volunteer to assemble the finished "tiles"
otherwise it's going to be all over the place.