[exim] Filter user variables outside of filters?

Does anyone know why filter user variables ($n0-$n9) are not allowed to be expanded outside of filters to give the last value set to them in a filter?

I have a system-wide per-user filter run from a router that I would like in some way to return a value to the main configuration file. I am trying to add headers on a per-user basis based on a filter script. If I could set $n7 and then expand that later in a "headers_add" in a transport it seems like that would do the trick.

Is there any reasonable way to pass a value back from a filter that can be detected in a transport?

David