Thank you, everyone, for the suggestions.
I actually hit it with several. I think it's about as good as it's going to get, and I've been using the piece.
I since picked up a slightly smaller casserole in Blue Cornflower, which has some sort of slight discoloration inside. I've tried all of the above suggestions, as well as a couple that I found on the interwebs, and absolutely nothing seems to touch it. I hope it's not gross that I've also been using this one to cook in.
I figured, if the slight staining won't come off with everything I've thrown at it, it's probably not going to come off in whatever I'm cooking in it.
Barry