Perhaps the simplest way to integrate external data dynamically is by creating an Access Script.

The Access Script is a DbfScript that intercepts any query to the database in that section. It is invoked whenever a list is queried. It then has the opportunity to update the actual list that is stored in the database from an external data source, to ensure it's in sync. The query then continues to retrieve the values from the database. The script isn't invoked when a specific record is queried, instead the query goes straight to the database.

To create an Access Script, go to the field definition record, then go to Advanced / External Data. Create this record, make sure Enabled is set to True and enter the script in Access Script that will update the database.

When this script is executing, any queries to the same area of the database will go directly to the database and won't invoke the script again.

The script doesn't return anything. It simply changes the database before the actual query runs.