Interesting post, its an age old question.
Legality or practicality.
The most important thing is protecting your ass! Whatever you do, you have to the able to make that decision being able to justify your actions. (Mainly to the local avaition authority, the boss & the insurance company perhaps). That said, an understanding of the rules, the company ops manual and flight manual may help because how can you argue against the rule if you do not know the rule to start with.
In the australian Civil Aviation Orders (CAO's), Part 95, Section 95.7, paragraph 7.2 it states;
The exemption, "from general requirement for a pilot to be at controls", in relation to a helicopter is subject to the condition that a pilot must, from the time of starting the engine or engines until the time of stopping the engine or engines at the end of the flight, be at the controls of the helicopter unless:
a) the helciopter is fitted with skid type landing gear; and
b) the helicopter is fitted with a serviceable means of locking the cyclic and collective controls; and
d) the pilot considers that his or her abscence from the cockpit is essential to the safety of the helicopter or of the persons on, or in the vicinity of, the helicopter; and
e) the pilot remains in the immediate vicinity of the helicopter.
Now, after all that the real questions are the definitions of "locking the cyclic & collective", "essential to the safety", "vicinity" and "immediate vicinity".
I know of CASA FOI's that have made a statement that "hot refueling" by the pilot is not "essential to safety" so this ruling does not apply (but that's another argument for another day).
But the definition of "essential to the safety", of the helicopter, and while remaining within the "immediate vicinity" to close a door probably is within the terms of the ruling as long as you can justify it.
Another thought; Why don't you brief your pax thoroughly and test them on opening and closing the doors prior to the flight?
Happy pondering.