[Exim] Modifying Subject header for some recipients only

Hi folks,

I need to modify the Subject header for certain local recipients only.

We use SpamAssassin, which adds an X-Spam-Score that any half-decent MUA
can use for filtering.

Some staff have gotten permission to filter spam, but use a crap MUA.
So they want [SPAM] prefixed to the Subject header, and they'll filter
matching messages into a spam folder, to save the odd false positive.

Other staff are not allowed to do any spam filtering, and management
won't allow Subject header tagging for them.

So, a system filter is out, and user filters don't allow the use of
"headers add" and "headers remove" (although I'm not sure _why_ not).

What do I do?

Ciao,
Sheldon.