gistfile1.cs

public class Item {
   public ReactiveCommand Remove{ get; protected set; }
}
public class MyModel {
public ObservableCollection<Item> Items { get; protected set; }
}

MyModel model = new MyModel{Items = { new Item(), new Item() }};
this.Items = model.Items.CreateDerivedCollection(projectFunction);
// how to wire existing Item's RemoveIt command here? 
// I had been enumerating the initial collection but it was ugly 
// and was hoping for a better way.
//
// this.Items.ForEach(item => item.RemoveIt.Subscribe(_ => Items.Remove(item)));

this.WhenAny(x => x.Items, x => x.Value)
    .Subscribe(c => c.ItemsAdded.Subscribe(item => item.RemoveIt.Subscribe(_ => Items.Remove(item))));