Your question is fascinating and I hope inspires people to think about pathophysiology and why we as physios apply certain techniques as part of our treatment strategies.
I guess from my perspective it would depend on the mode of ventilation in use, what the underlying ventilatory problem was and why it was felt necessary to apply MHI. If a patient is on PCV for example then I would question the role of disconnecting the pt from the ventilator to apply a manual technique (MHI) that is historically a volume cycled manoevre.
Even with a manometer in the MHI circuit you are still only provided with a peak pressure as a point of feedback - I think that to limit the peak pressure to 30cmH2O during MHI may result in a MHI technique that may not be effective and undoubtably delivering a volume that is lower than the Vt from the ventilator from a plateau pressure of 30. What does this add to patient care?
To answer your question I think, on a case by case basis, it would be useful to look at any variation in peak and plateau pressures along with their absolute values and Vt delivered to gain an appreciation of the lung compliance before deciding if MHI has a role. This of course assumes that there is an indication such as airway clearance or decreased ventilation that you wish to target.
If for argument sake, the decision is to provide MHI in someone with plateau pressures of about 30, do we still provide MHI within the historical constraints of keeping peak pressure below 40cmH2O?