I think a bearing guided bit would work but that wasn't quite what I had in mind. I'd try a chamfer bit with the depth set slightly less than the depth of the epoxy. Run the fence of the router along the edge of the steel. You'd probably need a couple of passes as the leading edge of the router would ride up a little but I'd guess it'd take longer to set up the router than it did to clean up the epoxy. I think this would probably work better with the more often used clamping-wood-to-the-steel method of keeping the epoxy in while it's poured as you wouldn't have to be quite so careful about correctly setting the depth of cut.

Great looking result Eddy.