When trying to set the message body text (the Comment field), we got the error:
One of the extension parameters is not valid for the following reason: you do not have permission to modify the value for the "Comment"The users all have the browser role, which allows Manage Individual Subscriptions, but this is not enough. There is a restriction in that setting that disallows the user from setting the comment field.
What I had to do was give the browser role the ability to Manage All Subscriptions. The major caveat with this setting is that if the application calls ListSubscriptions(null, null), the user will see all subscriptions in the system. We had to setup code on the front end to ALWAYS pass the UserID to the ListSubscription method. (http://msdn.microsoft.com/en-us/library/reportservice2005.reportingservice2005.listsubscriptions)
Also, note that for the My Reports folder, the browser security role is not used, its the My Reports role. You will need to give the My Reports role the Manage All Subscriptions task as well.