I think there are two options.
1 - Show the cursor at the full frame center and ignore the possibility the cursor will be not shown. If the bulls eye is drawn with center lines that extend the width and height of the full frame then the sub-frame has a good chance of having at least a part of the full frame graphic in view. The possibility of a cursor out of frame should not be a mystery to the user because they drew the sub frame and would know that the cursor is not in the sub frame.
2 - Allow the user to draw a centered sub-frame. Following the example of other drawing programs, if the shift key is down, the frame drag expands symmetrically around the center. This option solves the problem and allows the current behavior to remain. However, what is the usefulness of the bulls eye centered on the sub frame? What does it represent?
I think adding a second marker on the screen would be confusing.