Managed Direct requires FullTrust. This effectly means that running an MDX app in a web page is a non starter. Its just not a scenario the DirectX team considers a high priority. There is no way to work around this without creating a huge security problem on the computer and I can't recommend you even try to do this.
The only MS technology for doing 3d in a browser is the WPF stuff.
The otehr thing you could try is to build an ActiveX control - this way the security decision is handled by the end user who can choose to trust your application or not. However the responsibility is now all on you to ensure the code is fully safe and you also need to think about code signing dependign on your deployment