[exim] Logging of local interface for outbound mails

I have a server with multiple interfaces, more than one of which is used
for outbound SMTP according to various conditions.

Is there any way to log which local interface was used for the outbound
connection, on the "=>" log line? I've been looking in section 49.15 of
the 4.66 spec for a suitable log selector but I can't find one. There is
"incoming_interface" and "incoming_port" but, it seems, only the
corresponding "outgoing_port" and not the expected "outgoing_interface".

I appreciate that where an outbound interface is not explicitly
specified (e.g. the default configuration), Exim might not be able to
actively discover the outgoing interface from the OS. However where it
has been specified explicitly using the "interface" option in a
transport, it seems to make sense to be able to log that (ideally using
the "I=" notation to be consistent with incoming connections).

Thanks

Tim