This function is a part of the designator resolution mechanism, so it's not possible to access it directly in most cases, which makes an optional parameter useless.
In general, we have this kind of global flags for minor textual/graphical output quite a lot in the cram packages, I think it this looks quite ok. The advantage is that it's very transparent and easy to find.
What do you mean by a callback hook? Could you give a short example?