[Exim] "system filter" needs in a non system_filter filter

Hi All

I have a filter that I would like to run conditionally, based on a flag
set in my user ldap database. I have it set to run, but I would like
to be able to do things like set headers and stuff in it. Basically, I
have a spam filter that I want to run conditionally, depending on
whether a user wants it to be run or not, and if it is run, and thinks
it is spam, I want to add my own header into the message and then
continue with delivery.

However, the "header add" is only available for "system filters", which
seem to only be called by the filter specified in "system_filter". Is
there any way for me to set up a router that calls a filter that the
system will allow "system filter" commands in?

Thanks
Chad