Can a cast-iron casserole be used for steaming?
Enameled cast iron pots, like the one you have, can be treated as if they were steel for purposes of steaming.  You don't need to worry about rust unless the enamel is chipped or cracked. It'll take a bit longer to heat up, but it'll also retain heat better -- so keep in mind that it won't stop steaming until 15 minutes or so after you take it off heat.

If you had a non-enameled cast iron pot, I'd say that you shouldn't steam in it until you had a chance to build up a good seasoning. But that's not an issue for you.