[rt-users] Changing email subject to automatically update a ticket

[L B]

Hello,

 I'm going to receive emails with this subject "FOO BAR  Changeset : 1234 FOO".

 I want to include this message into ticket #1234 if it exists, or
create a new ticket if it doesn't (discarding this number, taking the
next free).

 I think the easiest way to do this, is changing the email subject
with a string at the beginning like [MY RT - #1234] so it will be
automatically processed by RT as a comment (or correspondance) of the
existing #1234 ticket if it exists, or not rename it if the ticket
doesn't exist (and thus the ticket will be normally created).

 How can I rename the subject of the email before a ticket gets created ?
 - before rt-mailgate ?
 - in rt-mailgate ?
 - in a scrip ?

 I would prefer a scrip because everything will be stored in the DB
and I don't need a script in the file system

If I do this in a scrip, I don't have to use "onCreate" condition, but
probably  "User defined" because I want to do this before the creation
of a ticket. But how can I get the object subject in the scrip as no
ticket has been created yet ?

Any other idea is welcome !

Thanks !
--
L.B.