I'm preparing a little Reflector Addin to be able to load and execute MbUnit fixtures inside Reflector. Since the fixture tree contains all the functionalities, it was just a matter of injecting it into Reflector.

Screenshot:

The source code is so short that I'm also publishing (note that I'm using the helper classes described here)

using System;
using System.Windows.Forms;
using Reflector.CodeModel;
using MbUnit.Forms;
namespace Reflector.Graph.Faulty
{ 
    // Reflector package
    public class MbUnitPackage : BasePackage
    {
        [ReflectorWindow("MbUnit")]
        [ReflectorCommandBar(CommandBarTarget.Assembly)]
        private MbUnitWindow MbUnit=new MbUnitWindow(); 
    }

    // Tree View
    public class MbUnitWindow : ReflectorTreeView
    {
        private ReflectorServices services =null;
        public MbUnitWindow()
        {
            this.Dock =DockStyle.Fill;
        }
        // needed to get reflector current element
        public ReflectorServices Services
        {
            get
            {
                return this.services;
            }
            set
            {
                if (this.services!=null)
                {
                    this.services.AssemblyBrowser.ActiveItemChanged-=new EventHandler(this.activeItem_Changed); 
                }
                this.services=value;
                if (this.services!=null)
                {
                    this.services.AssemblyBrowser.ActiveItemChanged+=new EventHandler(this.activeItem_Changed); 
                }
            }
        }
        private void activeItem_Changed(Object sender, EventArgs args)
        {
            this.RemoveAssemblies();
            IAssembly assembly = this.Services.ActiveAssembly;
            if (assembly!=null)
                this.Translate();
        }

        // populate tree with current assembly
        public void Translate()
        {
            IAssembly assembly =this.Services.ActiveAssembly;
            if (assembly==null)
                return;
            this.RemoveAssemblies();
            this.AddAssembly(assembly.Location);
            this.ThreadedPopulateTree();
        }
    }
}