Would it be too early for all the relevant agencies to get together and really develop these wayfinding systems up to a true global standard?
As you point out, Union Station might be on its way to becoming an international travel node. It might so happen that one day, not so far into the future, someone would pass through Union Station en route from say, Boston to Shanghai, or Lima to Ottawa. Sure it sounds farfetched right now but it can be surprising how people make use of new travel options that become available to them. There are many possible combinations that might involve one or both airports, the UPX, TTC streetcars, subways, GO trains and buses, etc. etc. - all connecting through Union Station (or YBZ, as we've now learned).
Since we have always aimed to "welcome the world" to Toronto, it might help to look at things through the eyes of a visiting Pan Am Games athlete, or a foreign exchange student, or any number of future international visitors who will come to the city for leisure, business or study. How might we develop the wayfinding at Union up to a level of clarity, consistency, and organization that truly serves the needs of an international customer base, and reflects its growing role as an international travel node.
(On a related note, one possible starting point for the various transit agencies is to consider updating their system maps to show both of Toronto's international airports and their connecting routes, both within and across various networks.)