In the Photography Collection at NYPL, we track geography in both Object and Constituent records. We do use the TGN for the Objects, so someone else may have suggestions for you there.* In the Constituents module, however, we are creating our own Authority resource for historical photographers, and capture as much information as we can about them (approaching 100,000 entries currently). Since our geographic information might be a studio address, a place of birth or death, or a general area in which the photographer worked, I use the Address field for them all. I have created Address Types for these different situations: Birth, Death, Active In, Studio or Commercial Address, Home Address....
To account for more general info (such as Expedition photographers) I have created some super-national entries such as Arctic Regions, South America (General).... I have reserved the Display Name 2 field for a display date.
This can get a bit tricky in that we also use these addresses for things like mailing addresses for living artists. For those, we use the Default Mailing etc. checkboxes and an Address Type something like INTERNAL USE Mailing, and the Display Name 2 is fair game for whatever info may be needed.
I have found it very useful in combination with Attributes, which we have some customized lists.
For instance I can do an advanced search for Addresses- End Date< 1870, then refine that with Query Assistant to search for Role: Photographer, Process: Daguerreotypes, Gender: Female, City: New York to find all the Female Daguerreotypists in NYC before 1870.
*I'd be interested in this, too, actually. Ultimately, I'd love to be able to plot our collections on maps, which would require LAT/LONG coordinates. I'd love to switch to something like OpenStreetMap or something someday....