This would be very useful to define meta types (E.g. TestFixtures, Test Assemblies, Domain Entities, MessageHandlers, DTOs etc.).
There is also the request for "Allow custom definitions similar to JustMyCode" which is the same in my opinion.

This should be implemented easily, as it is possible to do this today with a little hack:
/////// Rule that will be reused, defining test code ////////
// <Name>TestFixtures</Name>
warnif count > 0 // <= this is needed :(
from t in JustMyCode.Types

where t.HasAttribute("NUnit.Framework.TestFixtureAttribute".MatchType())

select t
///////////////
/////// Example rule that uses the other rule ////////
// <Name>Testfixtures should end with "Tests"</Name>
//let testFixtures = (from i in Issues where i.Rule.Name == "TestFixtures" select i.CodeElement).Cast<IType>()
let TestFixtures = Rules.First(r => r.Name == "TestFixtures").Issues().Select(i => i.CodeElement).OfType<IType>()
from t in TestFixtures

where !t.IsAbstract && !t.Name.EndsWith("Tests")

select t
///////////////
/////// Example rule for exclusion ////////
// <Name>Example of exclusion</Name>
let TestFixtures = Rules.First(r => r.Name == "TestFixtures").Issues().Select(i => i.CodeElement).OfType<IType>()

from t in JustMyCode.Types

where !t.IsAbstract && t.TypesUsingMe.Count() <= 0 && !TestFixtures.Contains(t)

select t
///////////////

Also defining custom extension methods may be even more usefull, as it fits the exclusion rule better.

This is a feature we'd like to provide, let's see how many votes it gets, this will help prioritize it.
Thanks