Mix flour, tapioca starch, baking powder, cornstarch, and 1 tablespoon rice flour in a bowl.
Add chilled soda water a bit at a time until you have the consistency of a thin pancake or crepe batter.
In a deep saucepot, heat 2 inches of canola oil to 350°F.
Dip pieces of lobster (and vegetables if using) into the remaining rice flour and shake off any excess. Then dip them into the tempura batter.
Place the battered pieces carefully into the hot oil. Be careful not to overcrowd the pan (6-8 pieces at a time). Fry until golden brown, flipping the tempura once or twice with a spoon.
Remove with a slotted spoon and drain on paper towels. Season with salt.
Serve with ponzu sauce or dressing of your choice.