Definitely decorative for your corbels...
You could install T-bar or angle 90 degree supports for the counter top and cover them with your corbels.
When you design the island, keep in mind your corbel design, then design the framing of the island to accept the mounting plan for the corbels.
Generally, the finish material or the counter, whether it be concrete or granite, is supported by a 3/4" plywood deck. The concrete deck can be formed with a recess which would hide the plywood. Your steel supports would then be fastened to the plywood bottom and through the island's vertical wall to the framing. You can design the corbels with holes for fastening to the plywood bottom and the framing components of the island walls.
(A year and a half ago I retired from my remodeling business in which I designed and built high-end kitchens and bathrooms)
The Kitchen pic is of my kitchen which I designed and built 20 years ago. The corbel pic shows wrought iron support for the cantilevered granite deck. You could make the supports from as noted above and cover them with your sculptures.