Filters...

RequiresSiteAttribute.cs

public class RequiresSiteAttribute : ModifyChainAttribute
{
    public override void Alter(ActionCall call)
    {
        call.AddBefore(ActionCall.For<SiteFilter>(x => x.HasSite()));
    }
}

SiteFilter.cs

public class SiteFilter
{
    private readonly ISiteContext _context;

    public SiteFilter(ISiteContext context)
    {
        _context = context;
    }

    public FubuContinuation HasSite()
    {
        return _context.Current == null
                    ? FubuContinuation.RedirectTo(new SiteSelectionInput())
                    : FubuContinuation.NextBehavior();
    }
}