Hm... Perhaps there could be a column to the left that would contain a small flat button (I think it could be styled that way) that has an eye icon on it - it could be made rather inconspicious with minimum borders. Mind you, I have not looked into GUI capabilities when it comes down to modding, but for example Task List has an interesting little light/icon next to each task that can be clicked through, and it is quite uninvasive - not sure how such a flat look was achieved.
As for 1:1 relationship, not sure I understand that one fully - generally speaking you would want to see where the items need placement.
As for performance, if it causes issue, it probably wouldn't matter if it's on for 10 seconds or longer. Players will most likely close the ghosts window at some point (which should clear the arrows as well), or get annoyed by the sheer number of arrows (if it's a huge area).
P.S.
Just tossing some ideas around without (being able to) looking at code. :)