[exim] Altering Subject header in a user filter?

Hi,

I have looked through the documentation & I suspect that the following
is not possible, but I'd be grateful for confirmation that that's the
case or advice on how to do it.

I'm trying to set up a "cheapo" small-scale mailing list on a
shell/hosting account that uses Exim & where I can set up user filters
("~/.forward-NAME"). Is it possible to prefix the incoming Subject
header with "[list name]" before running a few "deliver" commands?

Thanks,
Adam