I have been meaning to do some testing of this technique in the electric kilns, but of course I haven't gotten around to doing it. The main problem to overcome is that in cone 6 oxidation the iron oxide doesn't flux out as much, and therefore doesn't show through and mix with the glaze as well. So instead of doing this with slip, I thought I'd try doing it with layered glazes, using the technique I've been using to put stripes on my pots:
I am VERY happy with the results, and plan to do more. I also did another one with different glazes that did not turn out nearly as well. The top glaze was too stiff and didn't mingle with the bottom glaze very well. So now I have a good idea of what glaze combinations will work the best. The only down side is that I can't do this with these glazes on a vertical pot, as they will run and blend and wipe out the design. So platters are it for doing this with glazes. But now I'm really excited to start testing slips!