How to texture paint across all objects that share the same material/image? This is a greybox project, so we shouldn't take in account detail or sustainability. Just something that will "work".  

I have several objects that share one UV map and one material. I used Smart UV project for all of them so that it would pack them in the same space. I made a new image and applied it to the material.
I can texture paint each object just fine. However, I need to paint across all of them at the same time, in the 3D viewport (not the image editor). As it is, I need to select each object, then go to Texture paint and I can see my previous strokes, but I need to see them all at the same time (I'm making relationship lines etc). How to do this?
Also, my texture doesn't appear at all in material mode, although I have connected the texture to the diffuse channel.
If they already share all the same UV space, then temporarily you could then join them together until the paint stage is done.