[exim] Dynamic received_header_text

This question is related to my other thread which deals with Exim being
used in a front-end / back-end configuration, with back-end machines
handling separate email domains. I thought it would be better to post a
separate thread, in case it would help someone find it one day.

I would like the Received header to be set to empty only for incoming
emails - so as not to break SPF checks on Spamassassin on back-end
servers. Is it possible to either:

1. Set 'received_header_text' to and empty string inside the
corresponding router or transport config - or global variables can't be
amended inside routers or transports?

2. Otherwise, configure 'received_header_text' to use some conditional
statements, which return an empty string if the message is an incoming
message. Does 'received_header_text' get re-evaluated every time email
is processed - or is it only evaluated once - so this wouldn't work?

Any suggestions much appreciated.