At least the bts and nmudiff tools’ manpages do not seem to contain
information on how to make these tools not send out the eMails
directly but save it into a file, like reportbug can do. This would
be most useful on systems that do not have the ability to send mails,
such as some VMs, chroots, etc.

It should not be assumed the user is familiar with mutt, either.
--- /etc/devscripts.conf ---

--- ~/.devscripts ---
Not present

$ bts -n user devscripts@packages.debian.org . usertag 566394 nmudiff
From: James Vega <jamessan@debian.org>
To: control@bugs.debian.org
Subject: user devscripts@packages.debian.org, usertagging 566394
Date: Sat, 23 Jan 2010 10:53:44 -0500
User-Agent: devscripts bts/2.10.61
Message-ID: <1264262024-1681-bts-jamessan@debian.org>

user devscripts@packages.debian.org
usertags 566394 nmudiff
thanks

You can easily redirect that to a file and send it later.

For nmudiff, you can set the sendmail command to "cat > safe_for_later".
This is non-obvious though and having a -n type option would definitely
be preferable.

This was brought up in #564268 and I have plans for making our MUA
selection more flexible.

James Vega dixit:

Ah ok, sorry, did not see it at first.

OK. Maybe offer the file redirection right in there as well, that
would fix this one for me.

Thanks!
//mirabilos