In a large bowl, mix together the teriyaki sauce, brown sugar, onion, and the juice from the can of pineapple rings. Refrigerate the rings to use later.
Add the pork shoulder to your slow cooker and pour the sauce mixture over the top. Place the lid on the slow cooker, and cook on low for 7-8 hours, or until pork is tender and cooked through.
Transfer the pork to a separate plate and use two forks to shred it.
Discard half the liquid from the slow cooker, then transfer the shredded pork back into the slow cooker to soak in the remaining liquid.
Place a pan over medium-high heat. Add the reserved pineapple slices and cook on both sides until slightly browned (about 5-7 minutes).
Assemble the sandwiches by placing a generous scoop of pulled pork on each bun, topping with a slice of the "grilled" pineapple, and if desired top it all off with homemade coleslaw.