Is there any simple way to replicate sections in CiviMail message templates?
At this year's Denver sprint or con, Veda consulting demoed a working extension. The core stuff is supported by the upstream editor itself, so I don't think any smarty hacks were required. That's the limit of my knowledge though — I wasn't there. Hopefully we'll get to see it for 4.7 if not before.

Here is the code:
https://github.com/veda-consulting/uk.co.vedaconsulting.quickbulkemail

It also seems to simplify bulk mailing, but does not appear to be compatible with 4.6.

In 2016, they developed an extension to integrate mosaico.io into civicrm. See more about it here: https://civicrm.org/blogs/parvez/a-new-beginning-for-civimail%E2%80%A6