I cannot duplicate the behavior you describe where clicking the right mouse button on the Mac cancels the left mouse drag operation.
Regardless, what you are describing is not possible to implement on the Mac. Low level mouse events are delivered to a Mac application much differently than they are delivered on Windows. On MS Windows, the application gets “mouse moved” messages, and that includes which buttons are pressed at the moment.
On the Mac, mouse messages are delivered as a variety of different messages. They are mouseMoved, mouseDown, mouseDragged, mouseUp, rightMouseDown, rightMouseDragged, rightMouseUp, and even more.
The mouseDown, mouseDragged, mouseUp messages are delivered when the “left” mouse button is pressed. Remember that the Mac originally had only one mouse button.
If you click and drag the left mouse button, Rhino gets a mouseDown message, followed by a number of mouseDragged messages. If you then additionally press the right mouse button, Rhino continues to get mouseDragged messages. If you then let go of only the left button, Rhino will suddenly start getting rightMouseDragged messages. There is never an indication of when both buttons are pressed.