[exim] router for bounces

I have access to two smart hosts, i only want to use one of them for
delivering my bounce messages (post exiscan of course)

Is the proper approach to add a header the message upon error, and
when it comes to to for the transport to be chosen to look for that
header?

Seems as though that doesnt work because the bounce is its own new
message as it were. Also its very intensive to configure this. Seems
like i am missing a specification keyword or varible to tells me the
message is a bounce.

Thanks very much