Prepare the seitan as directed below, making a double batch of it. Drain well, cool, and cut into 1-inch chunks. Gently squeeze any excess water out if possible.
Heat the oil in a large nonstick skillet on medium-high heat. Add the seitan and pan-fry until golden brown but still soft. Avoid letting the seitan become dry and hard. Set aside.
Mix the marinade ingredients together in a medium saucepan. Add the seitan and place on medium heat. Bring the sauce to a boil. Turn the heat to low and gently simmer for about 1 hour covered.
Stir often to ensure that the seitan doesn't stick. Cook the sauce until it's quite thick. (Add a little water if it becomes too thick and pastelike.) Remove from the heat and cool enough to handle.
To assemble the kebabs, alternate pieces of seitan, pineapple, and bell pepper on the skewers, leaving space at the base for holding the skewer. You should be able to fill 12 sticks with 2 pieces each of pineapple and bell pepper and 3-4 pieces of seitan.
If barbecuing, place the skewers on a barbecue grill and cook until lightly browned, basting with the leftover sauce. If broiling, place the kebabs on a baking tray and baste each kebab with the remaining sauce. Broil until browned on the edges, turning to cook evenly.