Make the sesame-miso sauce: In a small skillet, toast the sesame seeds over moderately-low heat until golden brown and fragrant, about 10 minutes. Stir frequently and keep an eye on them, as they can burn easily.
Once toasted, transfer the sesame seeds to a blender or food processor, and add the miso, mirin, rice vinegar, agave syrup, and ½ cup water. Purée until smooth, a minute or two, adding more water if needed (the sauce should have the consistency of a creamy salad dressing). Just before serving, swirl in the sesame oil.
In a medium bowl, whisk the mayonnaise with the chili paste, tamari, and rice vinegar. Season with salt. Gently squeeze any excess moisture from the julienned cucumber and add to the dressing along with the crab and chives. Gently fold together until everything is evenly coated with sauce.
Let guests assemble their own tacos, layering the nori with rice and crab salad. Garnish with julienned bell pepper and carrots, along with a small dollop of masago, if desired. Eat immediately, passing the sesame-miso sauce at the table for drizzling or dipping.