If I understand what you are wanting to do, all I can think of, off the top of my head, is you'd have to create the fdr using the 8 directions in arc hydro and then use the raster calculator to convert the diagonal grid values to some other horizontal of vertical (H/V) grid value. I don't know how you would make the decision as to which H/V to convert to which diagonal values. I assume you may get some very strange results.
