Hey @DrewFlater, a follow up to this. I just tried to use arcpy's AddField with a "BIGINTEGER" data type, and ran into a similar issue as above:
ERROR 000800: The value is not a member of TEXT | FLOAT | DOUBLE | SHORT | LONG | DATE | BLOB | RASTER | GUID.
Failed to execute (AddField).
I'm not going to open a Pro Idea because... this issue of new data types clearly hasn't been addressed in a lot of places, so the "idea" would extremely general. I'll leave the prioritization decisions to you all. Thanks again for the initial reply.