To expand on what I mentioned in my post a little further up the page:
If you cannot insert the caret at the desired location, insert it somewhere nearby and then use the arrow keys to move it to another location - up or down to move onto another staff in the same system, or left and right to move into another rhythmic position in the same staff.
When it is where you want it, use the Clef popover.
This procedure might not be exactly what you are asking for, but it works.