In the U.S., Is it ever legal for an aircraft to occupy the runway turnaround area while another is taking off or landing? In voretaq7's answer to the question Is there a name for the area for aircraft to back track near the runway threshold?, it is mentioned that runway turnaround areas are considered part of the runway to which they are attached and that any aircraft in the turnaround zone are considered to occupy the runway.

I noticed that in the picture provided in his answer showing a runway turnaround area, the runway edge stripes actually included the runway turnaround area:



However, I've also noted that at the small, uncontrolled GA airport I've most often flown from, the runway edge stripes do not include the turnaround area:



Of course, in either case, a departing aircraft must cross the runway hold line and enter the runway to back-taxi to the turnaround area in the first place. However, I'm curious under what, if any, circumstances is it legal under the U.S. FARs for an aircraft to land or take off on a runway while another aircraft occupies the turn-around zone on that runway? And, of course, I'm talking about normal operations here, not emergencies.


Does the runway edge stripe configuration matter, for example?
How does this differ between controlled and uncontrolled fields?


For the sake of simplicity, let's assume that both aircraft in question are light GA aircraft, not aircraft whose wings might stick off the sides of the runway, and that all parts of the aircraft in the turnaround zone are on the turnaround side of where the runway edge line would be were it a straight line (like it is in the second picture.)

I have seen both landings and takeoffs with another aircraft in the turnaround area at the bottom field pictured above, including a couple of aircraft back-taxiing out there (under mutual agreement and while in radio communication with each other) and the second of them wait in the turnaround while the first takes off.
Jim's answer is accurate. Unless there is a proper boundary between the turnaround and the runway, an aircraft should not be there while other aircraft are landing or taking off.

In the FAA's AC about airport design:


  Obstacle Free Zone (OFZ). The OFZ is the three-dimensional airspace along the runway and extended runway centerline that is required to be clear of obstacles for protection for aircraft landing or taking off from the runway and for missed approaches.


The dimensions of the zone are different depending on the aircraft size and approach minimums. Specifically the runway OFZ has widths from 120 to 400 feet. This is the zone typically protected by the hold lines (pictured in Jim's answer), which are typically placed to provide adequate separation for the worst anticipated case at that airport.

Regarding the edge markings, they should be parallel to each other and not follow the edge of a turnaround pad. From the AC on airport markings:


  When a taxiway connects to a runway or the runway has turn pads or turnarounds, the runway edge marking remains continuous between the runway and these adjoining infrastructures.


If the turnaround pad is designed to be occupied while the runway is in use, this is referred to as a holding bay. A proper holding bay would look like this:



This illustration comes from the airport design AC. Notice that the FAA considers this poor, because it provides no guidance such as taxiway center lines for pilots to follow.