objectdef.cs

            var foo = new ObjectDef(typeof(Foo)); // you can also target a value instead of a type (new Foo())
            foo.DependencyByType(typeof(IBar), typeof(Bar)); // by type
            foo.DependencyByValue(typeof(IBaz), new Baz()); // by value

            Services(x =>
            {
                x.AddService(typeof(IFoo), foo);
            });

Would that work in a test assembly? I mean, without the full bootstrapping. What it has is just:

        var registry = new FubuRegistry();
        ClassUnderTest.As<IFubuRegistryExtension>().Configure(registry);
        _services = registry.BuildLightGraph().Services;

So, if I even add an IActivator, would it be.. activated?

Uhmm, in that case you need to walk through the services. I think it accepts a visitor so you can evaluate the plugin graph by yourself, but the rules dictating if is or if is not a valid plugin would be up to you.
If you also want to check that in your live app, you can also use an activator, just take in the BehaviorGraph which is IoC enabled in fubu and do the same check you do in your tests.
You probably are better asking this to JDM, he may give you a proper advice ;)

Ok, thanks for your help!