Nhibernate QueryOver. OrderBy using strings property names.

I'm refactoring old-style query CreateCriteria() to QueryOver(). My Wcf service gets string PropertyName to order queries results. For IQueryable I use Dynamic LINQ to do such ordering, for CreateCriteria() – AddOrder().

IList<object[]> result =
            GetSession()
                .QueryOver(() => activity)
                .JoinAlias(() => activity.ActivityLicense, () => license)
                .Select(Projections.ProjectionList()
                            .Add(Projections.Count<Activity>(e => e.Id), "ActivityCount")
                            .Add(Projections.Group(() => license.SerialNumber), "SerialNumber")
                            .Add(Projections.Count<Activity>(e => e.MacAdress), "MacAddressCount")
                            .Add(Projections.Count<Activity>(e => e.IpAdress), "IpAddressCount")
                )
                .OrderByAlias("ActivityCount") // Compilation Error - I need such extension method
                .List<object[]>();

Any suggestions how to do ordering in case with string property names?

PS: I could not use LINQ to Nhibernate: LINQ to NHibernate – .GroupBy().Skip().Take() cause an exception

Thanks!