Yes. As part of the notification, you pass a click context object.
This object can be any property-list object (dictionary, array,
string, data, number, or date). Remember that the container types can
only contain other property-lists: an array of NSImages is not a
valid property list.
So, the immediate solution is to take the absoluteString of the URL,
and in your delegate's click callback, re-create the NSURL from the
string.
If you have other notifications that you'd like to use strings for,
you can use the more general solution: set a dictionary as the click
context, and put the URL absoluteString into it. You can use any key
you want.