With the caveat that I am very much not an expert on this stuff, I suspect that you would get issues with scaling and warping of the metal, possibly even with the metal burning up. Keeping a gas-tight seal between the plenum and the flame face would be difficult if you had insulating board in between. As someone said, "If there's an obvious and easy solution, there's probably a good reason that people aren't doing it."
I would still recommend casting the refractory block. It may be a bit more pricey up front, but worth it in the long run. If you only have weekends available for shop work, would you rather spend that constantly maintaining and repairing your burner, or forging?