[pbarnes_1]

DKIM only signs headers. This is bunk. Move along.

[agwa]

You're wrong. As you can see on page 31 of RFC6376, the signature clearly covers a hash of the message body:

https://tools.ietf.org/html/rfc6376#page-31

It's possible to hash only a prefix of the body by specifying the 'l' parameter in the DKIM header field, but this is unusual and the email used as example in this blog post does not use the 'l' parameter.