[Exim] How to add content to mail with a filter

Hi,

I would like to add content to certain mails as they pass through Exim, the
need is as follows:

We have a support department and we keep an HTML catalogue of all of the
answered support cases that is accessible by an internal web-based search
engine. For all support mails that are not replies in an existing thread I
would like to build up a URL for the search engine based around the subject
of the mail and attach it to the mail so that when a support engineer looks
at the mail they will see an attachment that points to this URL which when
clicked, will open up a browser showing suggested solutions to the
customer's problem.

I've looked at the filter documentation and have also built and used several
simple filters in the past but nothing that attempted to manipulate messages
to this extent so I could do with a couple of pointers to get me started
assuming that it is possible to do the above.

I'm guessing that it might be necessary to shell out to an external script
and if this is the case I will need to 'clean' the subject of the mails to
ensure that they can't be used to execute commands on the server, if anyone
knows of any command-line utils that will save having to assemble a big sed
command then that would also be very helpful.

Thanks for you help with this, best regards,

Joe