Can I mount kitchen island countertop supports to the cabinet back panel?
Corbels are commonly used (as opposed to L-brackets), and they should work fine with your cabinet panel, assuming appropriate screws are used. Most of the force applied is in shear (up and down). 

You could also install a sheet of 3/4" BC plywood onto your cabinet (nicer side down), and lay the laminate top onto that. The plywood should fit neatly inside the laminate top's edge lip, and it will provide more than enough strength for your small overhang while maintaining a floating appearance.