With the lua_fix experimental build, it's as simple as:
-- Dual ISO bracket
menu.set("Expo", "Dual ISO", 1)
camera.iso = 100
menu.set("Dual ISO", "Recovery ISO", "1600")
msleep(500) -- fixme - settings are not applied right away
camera.shoot()
menu.set("Expo", "Dual ISO", 0)
msleep(500) -- fixme - settings are not applied right away
camera.iso = 100
camera.shoot()
camera.iso = 1600
camera.shoot()