I would seek guidance from the patient's oncologist/primary physician in this case, to clarify whether they were or were not happy for you to proceed with physio. I would say also that there is very little point in performing "slight" chest physio - to my mind you either treat fully or you don't treat at all. Positive pressure devices may be more effective than manual techniques perhaps? Also think about why the patient is unable to expectorate secretions - if it because they are tenacious and difficult to clear from distal airways, then physiotherapy is certainly indicated (with Dr's approval), but if the patient is unable to expectorate due to a physical obstruction in the trachea itself, then what is physiotherapy going to achieve?