I checked the msdn2 page for the Exec task (http://msdn2.microsoft.com/en-us/x8zx72cd.aspx) and it says:
One disadvantage of using the Exec task rather than a more specific task is that it cannot gather output from the tool or command that it runs.
At the risk of sounding silly, maybe you could pipe the output of the command to a file ( with | or > in your Exec command) and read from the file with the ReadLinesFromFile task?
Stephane