I've created custom races based off another, but I'd still like to be recognized as "argonian" or "redguard" in the intro and by guards in the open world. Any ideas where I can find what controls how race is recognized by NPCs?

It's almost certainly part of the MAIN QUEST.

Check out the Dialogue View of that to see how they forked it for race

(I'm guessing, but I'm fairly certain it will be done that way)

Thanks. It looks like the game is calling GetPCRace ArgonianRace=1 for each individual quest. Now I need a way to set my custom race as an alias for the main race, if that is even possible.