[exim] caseful $local_part in transport

Hello,
after studying the Exim documentation I found information about
how $local_part could be handled casefully by setting the
caseful_local_part parameter in a router configuration or by
using the +caseful option during an address lookup.

Now I wonder how I could pass $local_part in caseful form to a
transport script. Any hints are appreciated. Is it possible at all?

Regards
Juergen