I double checked and the system still works - it modifies events when attendees say whether they will attend. This is because the Reply-To is unchanged. Again, the script already attempts to set the From address to the email for the organizer - I just added a second way to determine what the organizer's email is. BTW, my method requires that an email is defined for the user in Workgroup Manager. I still don't know what would cause the info which the script looks for by default to be present.

Of course, I suppose someone could get the invite and instead of saying yes or no, hit reply to the email, expecting their reply to make it back to the organizer. But, having a recognizable "From" seems better than receiving an email from an obscure ("com.apple....") email address...