Can I support a granite countertop overhang with embedded steel bars?
Based on the flexibility of the flat bar stock, I decided to use square 1/2" tube stock.

Using a 3/4" MDF subtop, I routed channels 5/8" wide and 9/16" deep. I then filled these channels with polyurethane glue, set in the steel bars, and finally I used a drywall knife to spread and flatten glue over the top of the bars. 

Here are pictures of my actual work: 
The cabinets are backed by a 2x8 pony wall. The MDF is glued and screwed to the pony wall and all cabinet frames. The steel bars span everywhere except the sink cabinet.