Developing Zimlet to change subject and message text. A question. Is possible to Index Meta content?

[Javier Garcia Alarcon]

Hi all 
One of our customers have requested to have the possibility to change the subject of a received email. They want to add some extra text, so in the future when performing a search they can find the emails easily with the text they have added to the subject of those emails.
We have been playing around with the API, creating a Zimlet and we have been able to get the email from the server, change the subject locally, but we couldn't find any way to save that changes in the server. We also took a look into the java API and could not find a way to achieve what they've requested. So the question is... is it possible to change the content of a received email and save the changes in the server?
 
If the last question is not possible to accomplish, we know that is possible to add some custom meta content in the email and save it in the server. However when performing a search, the search doesn't "look" into that content. So, is it possible to perform a email search, also looking for text in the custom meta content?
 
Thanks in advance for any information and best regards.