One of the features that I have been asked for multiple times is the ability to use commands at time. I have now implemented this with the following syntax:

Package: foo
Version: 1.0-3
Control: retitle -1 this is the title
Control: severity -1 bleargh
Control: summary -1 0
Control: forward -1

In short, you preface any control commands with Control:, -1 is the current bug, and the rest of each line is the control@ grammar you already know. This also now works for every kind of message to with the exception of messages received at nnn-done and nnn-forwarded. I don't know why you'd use it for anything else but submit@ messages, but hey, whatever works.