My understanding from the author of TD.NET is it is very difficult to intergrate Reflector with Visual Studio. He use to provide a free Reflector Add-in but (I don't remember his reason) decided to roll it up into newer versions of TD.NET. I believe the source code (ReflectorVsAddin.zip) for the original add-in is still out there. (Found the old .NET 1.1 / Visual Studio 2003 souce code, see Update below.) Someone probably has the original zip file.
To use with older versions of Visual Studio, I still have either all or some of the files included in ReflectorVsAddin.zip. I am sure some hardy soul could disassemble them and create an add-in based on the techniques. Yet, searching on "Reflector Hosting" resulted in numerous articles on how to integrate Reflector with your own application. I believe it would be easier to start from scratch. I personally, find using .NET Reflector externally outside of Visual Studio easier to use.
Update
Building the Reflector Add-In
By Jamie Cansdale
Making Reflector into a Visual Studio.NET Add-In
http://www.codeproject.com/KB/macros/reflectoraddin.aspx
Update
How to: Load Assemblies into the Reflection-Only Context at http://msdn.microsoft.com/en-us/library/ms172331.aspx.