For the kalua pork: Prepare a grill for medium-high heat. Season the pork with the salt and chipotle. Grill the pork until charred, 4 to 5 minutes per side. Transfer the pork, onion, jalapeno and pineapple to a pressure cooker and cook for 35 minutes.
For the pineapple rice: Bring the rice and 2 cups water to a boil in a pot. Turn the heat down to low, cover and simmer for 20 minutes.
In a saute pan over medium-high heat, add the Spam and cook until it gets some color and crisps up a bit, about 5 minutes. Then, add the onions and cook for 2 minutes. Add the pineapple and cook for 2 more minutes.
Once the rice is done, fold it into the pineapple mixture and season with the vinegar, salt and pepper.
For the unagi sauce: Bring the mirin and sake to a boil in a saucepan. Add the sugar and cook until dissolved. Then add the soy sauce and simmer for 20 minutes. Remove from the heat and let cool for a few minutes, then mix 1/4 cup of the unagi sauce with the mayonnaise.
To assemble the bowl: Place the pineapple rice on the bottom of a bowl. Place some of the cooked pork on top of the rice. Add a dollop of the unagi mayonnaise over the pork and garnish with green onions and toasted black sesame seeds.