You may want to play with calling the sp using isql or osql (see bellow pulled from T-SQL help file):
Using the osql Utility (isql could be used too)
The osql utility is a Microsoft® Win32® command prompt utility for ad hoc, interactive execution of Transact-SQL statements and scripts. To use osql, users must understand Transact-SQL.
The osql utility is typically used in these ways:
Users interactively enter Transact-SQL statements in a manner similar to working on the command prompt. The results are displayed in the command prompt window.
Users submit an osql job either specifying a single Transact-SQL statement to execute or pointing the utility to a text file that contains Transact-SQL statements to execute. The output is usually directed to a text file, but it also can be displayed in the command prompt window.
Other options are that you could build an Active X control that you could instantiate in the sp and feed it the output parameters having the Active X control assemble the resulting text file and output it using the file system object.