[Exim] Adding headers in user filters

Hi.

I'm trying to create web-based system for users, that (among other
things) allows them to manipulate their filters. Filters (with other
users data) are stored in postgresql database.
So far so good - everything works fine....

but users should be able to type any type of filter (autoresponder,
autoforward, simple logging etc). Some of them generate a new mail. And
that is the problem. It the text contains some national characters there
should be added proper Content-type header. As the filter is typed via
web-form I could easily check if it contains any national chars and if
so modify properly the filter to add the header.
Unfortunatelly the simple headres add won't work as it's a user filter.
Is there any way I can circumvent this?

regards
Przem