In the April-June 2006 issue of Cognos SupportLink (volume 16 number 2, starting on page 77) there is an article that describes how to accomplish this using an HTML item on a prompt page and some java script code. Esentially this technique would allow a report consumer to run a report, and a button on the prompt page could be clicked to download the text file and populate the values in the prompt.
I am not sure if the text file was comma delimited or there was a distinct value on each line of the text file. Let me know if you do not have access to the SupportLink document and I can attach it.