[exim] Tweak username in transport for dspam

I'm using Exim 4.44 with Dspam 3.5.0. Messages addressed to my users can
be of the form user@domain or alias@domain, where alias is normally a
fullname (jane.doe). The dspam router checks availability
of a service attribute via LDAP for that user to determine if the user
has opted in for Dspam.

dspam_router:
   no_verify
   condition   = "${if and { \
                           {!def:h_X-Spam-Flag:} \
                           {!def:h_X-FILTER-DSPAM:} \
                           {!eq {$received_protocol}{local}} \
                           { <= {$message_size}{512k}} \
                           {!eq {${lookup ldap {ldaps:///BASEDN?uid?sub?\
                                   (&(|(mail=${quote_ldap:$local_part@$domain})(alias=${quote_ldap:$local_part}))(service=dspam)) \
                                   } {$value} {}}\
                                } {} } \
                           }\
                           {1}{0}}"
   headers_add  = "X-FILTER-DSPAM: by $primary_hostname on $tod_full"
   driver       = accept
   transport    = dspam_spamcheck

Unfortunately, at the point of the transport, the value of `%u' can either be
`userid' or `alias', depending on the $local_part of the message's address.

dspam_spamcheck:
  driver = pipe
  command = "/usr/local/bin/dspam --deliver=innocent,spam --user ${lc:$local_part} -f \
           '$sender_address' -- %u"
  home_directory = "/tmp"
  current_directory = "/tmp"
  user = exim
  group = mail
  log_output = true
  return_fail_output = true
  return_path_add = false
  message_prefix =
  message_suffix =

How can I ensure that the command in the transport is called with the userid as
retrieved from LDAP (i.e. I always want the userid, even if the message was
sent to alias@domain) ? I can't think of anything short of using a command= which
would once again perform the directory search and then launch dspam; that can't
be right.

Thanks for any help,
    -JP