I'll answer this question with an answer that does not answer your question. I think you are looking for a solution that is looking for a problem that, if you found one, will create a lot of problems.
A PM must be authorized and enabled to add and delete tasks and move around resources as necessary to deliver the project scope. By adding a layer of control on this normal PM action, then your PM is not really the PM. This signals to me that either you are under hiring your PM and need to raise the requirements for this role or you have a PM maturity capability issue or a cultural one or both.
So long as the added tasks are not associated to scope creep--e.g., your scope is to paint the bathroom but you are adding tasks to paint the living room--then the PM should be able to add additional coats of paint to the bathroom, and commit the paint, the painter, money, whatever else, to complete the painted wall. In this case, the baseline schedule--including its planned value duration and money and hours--do not change; thus, you now have a variance that you must manage. Variances are normal. Trying to eliminate them by control points is futile and will very likely exacerbate variances.
If the PM wants to add the living room to be painted, then this is where your change control process comes in to play, a proper control point. Upon approval, you get a new schedule, new money, new time, and a new baseline and the work continues.
So my recommendation would be to change your requirements and remove this need for a control point under this scenario. Find out why they are requiring it and go fix it. Fixing the root will be much more beneficial than a band aid control point that will likely cause adverse effects.