FWIW, there are people installing granite now with no ply under.
Recently I saw a 2cm thick countertop that stuck out 24" -- bare and unsupported -- and it was finally supported on one leg centered near the end. Nothing else. Obviously nobody is ever going to get up on it, because it looks too flimsy.
Honey, getting back to your case: I saw you asked near the begining "Do you think fabricators sell these things? Are braces attached the same way as wooden/metal corbels?"
Just to be sure we are helping you, I'll generalize your requirement. Here goes: any triangle is a brace (a corbel is a brace), and any two screws, one at each end, will hold the (triangular) brace in place, strong enough to do the job necessary. It's the triangle that does the job. It just happens that steel and some other metals are so strong that the triangle can become almost invisible to "you", since its shape looks more like a stick sticking out than a formal triangle. The three points of the triangle are still there, but flattened into a 1" or 2" bar or tube or "flat stock".
Any support will work. Any triangle, whether it looks like an empty triangle or not, whether a bar, tube, wood beam or whatever.
Your distance to support is short.
Your kids may promise never to sit on it or stress it with sudden shocks, blows and bumps, in which case you could leave it unsupported. Ha.
david