If you look at your Xcode's AppKit/NSPopover.h, you'll see this in the NSPopover object:
NS_CLASS_AVAILABLE(10_7, NA)
@interface NSPopover : NSResponder {
@private
...
...
id <NSPopoverDelegate> _delegate;
...
...
NSView *_positioningView;
...
...
So the popover object keeps a pointer to its positioning view but also keeps it private, so it won't be available to subclasses.
One thing you could possibly do is write a category extension to NSPopover to add an API to return _positioningView.