Mix the sauce well. Slice the seitan loaf 1/8 to 1/4 inch thick slices.
Spread some sauce about the shape of the seitan loaf on a sheet of aluminum foil large enough to fold over and enclose the seitan.
Spread open each slice and slather on the sauce. Close it back up and spread more sauce over the top. Fold the edge of the foil over several times and seal the ends well. Wrap it twice so you don't have a sauce blowout.
Cook it on the barbecue pit until the sauce inside is almost entirely absorbed and highly thickened. If you want a nice line of char on the bottom, don't turn it over halfway through. It's tough to say how much time because grill and fire pit temps vary wildly. The position on the grill over the heat matters as well. Last time I made it, it was on about 30 minutes but needed 45 to get the char, just to give you an idea. It was delicious anyway.