Cantilever Question - Model Railroader Magazine

If I understand you, you are using 1/4" plywood atop stringers of 3/4" ply that will be about 4" high...is this correct?  Are the stringers going to lie flat, or are they going to be vertical?  Will the tips of the brackets be fully supported by a gusset or a brace, or will there be a tongue with a hole for a screw projecting beyond the brace?

These questions are all relevant, but.....with 1/4" plywood, I wouldn't be worried so much about the braces and the weight.  I would be more worried, quite a bit actually, about you grabbing crepe paper plywood in a desperate bid to keep yourself righted and find you cracking it for several feet, and all those nice items cascading onto the floor.  For that reason, I would want top grade ply and a projection of maybe 4" beyond the eyelet-with-screw-hole.  Having such thin plywood projected much more than that is inviting great regret in the future. 

Now, if you could see your way to upping the ply to even 1/2", that would bring you out to the 12" you desire.  Then the problem would also include the anchoring closer to the wall to ensure you don't horse the whole shelf off of the braces and have it all come down.