If the background is not plain, I don’t know how you can do it. If it wasn’t chroma, you could conceivably try to key it, and using a variety of filter, it could work. But if the background is complex, with moving objects, especially if they are of various colors which may match those in the object you want to silhouette, I don’t know how you can do it.
Rotoscoping might be your only way. Tedious, yes, but you certainly wouldn’t be the first to have to do it. Some people make a living doing it every day.