I do not have C2 rigt now, but lets try.
first, if needed, you disable your list with the set enable action(as there is no choices until the text file is loaded)
Then you request the file with an ajax request, I will assume the file is in this format:
when the request is completed, you have access to the content of the text file with AJAX.LastData, so:
On Ajax Request completed
system>repeat tokencount(AJAX.LastData, "|") times
List>Add Item tokenAt(Ajax.LastData, loopindex, "|")
then you enable the control, that should work.