[freshprince]

I guess it would be something like this:

  T.<cr>;g

'T' means tag messages matching a given pattern. '.' would be the pattern that matches all messages in the current mailbox. So now all messages are tagged. ';' is the prefix for 'do the next command for every message that is tagged' and 'g' is reply to all.

[dsr_]

Mutt will then assemble everything into a single massively multipart mail message and bring it up in your editor so that you can write an additional message of your own. Then you have to save the edited file and OK mutt to send it.

I would not characterize this as a single command...