Herringbone pattern on a curved step?

Sure you don't want to re-think this? Sounds somewhere between being very difficult to impossible to pull off as well as being dangerous. (Not being visually clear.)

If it's a curved step you lay your tile and over-hang the edges and transfer the curve contour line and use a grinder following the line (If you can get a hold of Dremel, a model with most power with a diamond wheel on it, it will take longer but you will get a better and more precise cut) To finish it use diamond sandpaper to smooth out the rough edges so the molding profile will fit much better against it... As the trim goes like you said use Metal Radius Tile Edging, Schluter is probably the best choice because they have lots of finishing options.