Testing delays in Rx

I'm trying to work out how one could go about testing the following function, which adds monitoring around the internal queue of Observable.ObserveOn.

public IObservable<T> MonitorBuffer<T>(IObservable<T> source, Action<int> monitor, TimeSpan interval, IScheduler scheduler)
{
    return Observable.Create<T>(ob =>
    {
        int count = 0;

        return new CompositeDisposable(source
            .Do(_ => Interlocked.Increment(ref count))
            .ObserveOn(scheduler)
            .Do(_ => Interlocked.Decrement(ref count))
            .Subscribe(ob),
            Observable.Interval(interval, scheduler).Select(_ => count).DistinctUntilChanged().Subscribe(monitor)
        );
    });
}

I envisage something like this:

var ts = new TestScheduler();
var input = Enumerable.Range(1, 8).Select(i => OnNext(i * 10, i)).ToArray();
var hot = ts.CreateHotObservable(input);
var observer = ts.CreateObserver<int>();
var log = new Subject<int>();
var monitor = ts.CreateObserver<int>();
var ticks = TimeSpan.FromTicks(5);
var buffered = MonitorBuffer(hot, log.OnNext, ticks, ts);
log.Subscribe(monitor);

buffered.Do(x => { /*if(x == 3) Introduce delay here */}).Subscribe(observer);

ts.AdvanceTo(100);
observer.Messages.AssertEqual(...);
monitor.Messages.AssertEqual(...);

The question is, what can I put in the Do to get the desired effect of a temporary downstream delay.

I'm looking for results something like this:

//time:    0--------10--------20--------30--------40--------50--------60--------70--------
//source:  ---------1---------2---------3---------4---------5---------6---------7---------
//output:  ---------1---------2-----------------------------345-------6---------7---------
//log:     ----0-------------------------1---------2---------2----0-----------------------

(NB: I asked a similar question a while ago, but it wasn't very clear, and it's a bit late for a complete rewrite now).