[Exim] Conditional header rewriting

Hi,

I've recently installed EXIM and Cyrus to handle my company's email and
I have a question about the rewriting of headers:

Our support department share the incoming support mail via IMAP and so
anyone can reply to a support email from their own installation of
Netscape which is set up with their personal address rather than the
generic support one. We would like to rewrite outgoing messages so that
their From field shows support@??? rather than
joe_bloggs@??? but not always as joe bloggs will also want to send
personal and internal emails under his own address.

My only thought on how to solve this was to try and build a filter that
says if Subject contains some-strange-string then From equals
support@??? and Subject equals subject without
some-strange-string.

The first part of my question is "What is the best way to implement
this?"

The second part is "Does anyone have any examples that I could use as a
starting point?"

Thanks in advance for any help,

Joe