FWIW, here's how I did it:
1. Added LoadStructures (pulled in the JSON file), tagging it uniquely
2. Added a new event to check when that uniquely-tagged LoadStructures event finished
3. grabbed the .LastData value of the LoadStructures event (which was everything in the JSON file) and added it to my in-game array
So tagging the LoadStructures AJAX object was the answer, but *referencing* that was the hard part to figure out