Bring two cups of water to boil. Place the soy curls and bouillon in a mixing bowl. Pour the hot water over the soy curls and bouillon and stir well. Allow them to soak for 10 minutes.
Meanwhile, make the slaw. Combine the cabbage and apple in another large mixing bowl. Whisk together the oil, vinegar, agave or syrup, and salt. Pour these ingredients over the cabbage and apple. Toss well and season to taste with black pepper.
Gently warm the barbecue sauce in a medium saucepan over medium low heat. When the soy curls have soaked, drain them, press them gently to remove excess water, and transfer them to a cutting board. Give them a rough chop. Transfer the soy curls to the saucepan with BBQ sauce and heat them through, stirring them well so that they're evenly coated in sauce.
Optional: If you have an air fryer, at this point you can transfer the soy curls to the basket of your fryer and heat them at 350F for 8 minutes. They'll be a little drier and chewier than they were before.
To assemble your sandwiches, place about 1/2 cup of soy curls onto each bun, followed by a scoop of the slaw. Serve.