[exim] unseen and headers_add

The spec says "Warning 2: If the unseen option is set on the router,
all header additions are deleted when the address is passed on to
subsequent routers."

Drat.

I'd like to add a header (actually, modify the subject) in an unseen
router. What tricks are other people using to accomplish this? (I am
thinking of adding some junk to the redirect data so that I can
process it with some other router that won't match normal addresses.
Since I haven't tried this yet, I don't know if I'll be successful.)