[Exim] Localuser Permissions on Director Lookup?

I'm trying to get Exim to do a lookup using the localuser's permissions
on the director level (it does set the permissions to the user at the
transport level). Here's the director:

spam_email:
  driver = localuser
  transport = spam_delivery
  condition = "${if and {                                                      \
                    { def:header_x-spam-warning: }                             \
                    { or {                                                     \
                           { ! exists {${home}/.nospam} }                      \
                           { eq {spam} {${lookup {$sender_address} lsearch*    \
                                         {${home}/.nospam} {nospam} {spam}}} } \
                         }}                                                    \
                    } {yes} {no}}"
  suffix = -*
  suffix_optional

The problem is that ${home}/.nospam has 0600 permissions and setting
'user = ${local_part}' also only affects the transport level. Note
that 'root' on the fileserver does not have root permissions on ${home}
so that's not the solution either. I don't want to push off the lookup
to the transport level since if the lookup succeeds, I want to continue
to the next director.

Any pointers will be appreciated.
___________________________________________________________________________
Ephraim Silverberg, CSE System Group,        Phone number:    972-2-6585521
Hebrew University, Jerusalem, Israel.        Fax number:      972-2-6585439
WWW: http://www.cs.huji.ac.il/~ephraim       E-mail: ephraim@???