Sometimes works, sometimes doesn't. For instance hiring a car for 5 days in 7 has priced as a holiday for me with associated benefits of deposit, bonus Avios, G class availability etc.
Quite often the hotel is included at throwaway prices so you might not spend that much more even if you don't use all the nights.
For your exact itinerary you can check by doing flights only and adding the hotel when prompted in the booking processs. If it comes up with a BA Holidays-style page, you've won. If it comes up with the hotel as an add-on, then you might as well check adding the hotel for all nights.