The rice bread sando is the standard Japanese sando rebuilt on komeko pan, bread baked from rice flour instead of wheat. It exists as a gluten-free alternative, and it is honest to say up front that the swap is a real one with real trade-offs, not a like-for-like substitution dressed up as identical. The filling logic is unchanged, whether egg salad, fruit and cream, ham and cucumber, or katsu, but the bread underneath behaves differently enough that the sando is its own thing rather than a footnote to the wheat version. For people who avoid gluten, it is the difference between a sando being off the table entirely and being available at all, which is the point of it.
The craft is mostly a baking problem. Rice flour has no gluten, so the dough cannot build the elastic structure that gives shokupan its springy, foldable crumb; rice bread tends to be denser, moister, and more tender-crumbed, with a shorter window before it dries out and turns crumbly. The better loaves use fine-milled rice flour and added binders to hold a structure soft enough to slice thin without cracking, and they are best used fresh, since they stale and firm up faster than wheat bread. The filling has to suit that base: drier, well-bound fillings travel better than very wet ones, because a loose crumb soaks moisture quickly and can fall apart. Handled well, the result is soft, clean-tasting, and faintly sweet from the rice, a legitimate sando in its own right. Handled poorly, it is dense and gummy, or dry and fragile when the bread sat too long before it was filled.
The variations track the bread program more than the fillings. Some bakeries do a plain rice loaf; others blend in tapioca or psyllium for a chewier, more bread-like bite that comes closer to the wheat texture. The fillings span the usual range, from tamago and tuna to fruit and cream and katsu, each adapted to a more delicate crumb. The wider subject of gluten-free and allergen-conscious baking in Japan, and how the classic sando lineup is being re-engineered for it, is a substantial topic that deserves its own article rather than being crowded in here.