Ideally, I am looking for T-SQL that returns two columns: the node name, and the date/time that node was added to the availability group, for all nodes in a given availability group.
This is as far as I can see not possible with a single T-SQL statement as individual nodes hold only their own create date.
The sys.availability_replicas page makes reference to a column create_date. This is the datetime value of when the node was added.
To find the create_date for each replica you would need to first connect that replica and query the table for that instance only as below.
SELECT [replica_server_name], [create_date] FROM [master].[sys].[availability_replicas] WHERE replica_server_name = @@SERVERNAME