Possible to load a web part inside another?

No chance on getting the source for the WebPartFactory class is there? Or maybe a bit more information about it? Pseudo code maybe? If a custom web part is in the gallery it could be referenced in the same way as RSSViewer is correct? I'm just not really sure how to go about doing what you have done here, and I would very much like to better understand how to do this.

Thanks!

There are (at least) two ways to do this: using iframe HTML element, or just a div whose content is changed by JavaScript (probably with Ajax).

[NOTE] My answer is generic (ie. on Web design side), I have no idea how it in your technical context, so maybe I should delete this answer...

public class WebPartWrapper : System.Web.UI.WebControls.WebParts.WebPart {
    protected override void CreateChildControls() {    
        Panel pnl = new Panel();
        this.Controls.Add(pnl);
        var factory = new WebPartFactory()
        WebPart dynamicPart = factory.CreateWebPart("RSSViewer", this.Guid);
        pnl.Controls.Add(dynamicPart);
    }
}

public class WebPartFactory {
    public WebPart CreateWebpart(string webpartName, Guid parentWebPartGuid)
    {
        var config = ConfigurationFactory.LoadConfiguration(webpartName);

        Assembly webPartAssembly = Assembly.Load(config.Assembly);
        Type webPartType = webPartAssembly.GetType(config.Class);
        object actualWebPart = Activator.CreateInstance(webPartType);

        foreach (var item in config.Properties)
        {
            PropertyInfo webPartProperty = webPartType.GetProperty(item.Name);
            object webPartPropertyValue = Convert.ChangeType(itemValue, Type.GetType(item.Type));
            if (!String.IsNullOrEmpty(item.Value))
                webPartProperty.SetValue(actualWebPart, webPartPropertyValue, null);
        }

        RunMethod("set_StorageKeyInternal", actualWebPart, new object[] { parentWebPartGuid });
        return actualWebPart as WebPart;
    }

    private void RunMethod(string methodName, object objectInstance, object[] methodParameters)
    {
        BindingFlags flags = BindingFlags.Instance | BindingFlags.Public |
            BindingFlags.NonPublic;

        Type t = objectInstance.GetType();
        MethodInfo m = GetMethod(t, methodName, flags);
        if (m != null)
        {
            m.Invoke(objectInstance, methodParameters);
        }
    }

    private MethodInfo GetMethod(Type instanceType, string methodName, BindingFlags flags)
    {
        MethodInfo m = instanceType.GetMethod(methodName, flags);
        if (m != null)
        {
            return m;
        }

        if (instanceType.GetType() == typeof(object) || instanceType.BaseType == null)
        {
            return null;
        }

        return GetMethod(instanceType.BaseType, methodName, flags);
    } 
}

This code needs some explaining... Please excuse me if it does not compile, I had to remove a fair bit of the original code, it was very implementation specific stuff. I've not shown the "config" class either, it's just a container for configuration of webparts, just a bunch of properties. There are 2 issues I'd like to discuss in more detail:

1. parentWebPartGuid - This is the Guid (UniqueId?) of the hosting webpart. For some reason we have to set "StorageKeyInternal" to this value, using reflection (it's a private property). You can possibly get away with not setting it, but at least for the majority of webparts we had to set it.

2. config.Properties - This is the config values (we set them in a custom .xml file, but feel free to get this from anywhere). It can look a little like this..

In our framework we also support stuff like dynamic property values etc., but that's for another day... Hope this all makes sense and can help somebody.

I don't think so. I tried this a while back and it complained about only being able to add WebPartZone items in Page Init. I think by the time it get's to initialising your "container" WebPart it's too late to add more zones as the holding page has already been initialised.