I've said at many times this is possible, but have never had the cycles to go do it. (As a side note, there are actually a lot of things that are possible at the ICorDebug platform level but that the Visual Studio Debugger does not actually do. I've been meaning to blog more about this platform vs. application gap...)
So I'm excited to see somebody trying this out. The closest I got was adding the MDbg extension to debug IL. The MDbg source is public and uses public APIs, so brave folks could also try extending that sample to add Reflector support.
I've added Felice's project to the Mdbg link's page under "Other people's Mdbg work". If you've been using Mdbg to do cool tools like that, feel free to ping me and I'll add your tool's URL to that page too.
Update [5/8/06]: Something's available here: http://www.felicepollano.com/PermaLink,guid,1c863e69-1b56-4cfa-a60b-203e1127b8bd.aspx