[Exim] home_directory and filters

I'm building a system where users have no home directories, and
all per-user configuration is done using a mysql database. I've
got most of it working nicely now, but for one thing:

Using the following router:

userfilter:
driver = redirect
check_local_user
no_verify
no_expn
check_ancestor
allow_filter
data = ${lookup mysql {select filter from filters where user='$local_part' and toggle=1}{$value}}
file_transport = address_file
reply_transport = address_reply
directory_transport = address_file
transport_home_directory = /var/mail/$local_part

(a bunch of forbid_* items will be added later, this is still in the
basic development and testing stage.)

and the following transport:

address_file:
driver = appendfile
delivery_date_add
envelope_to_add
return_path_add
maildir_format = true
home_directory = /var/mail/$local_part

The home directory when processing filters is still set to whatever
comes out of NIS -- which, of course, does not exist.

This is only a problem in that I'd rather not have to require users to
specify the full path of their mailboxes when writing filters.

Anyone got any hints as to what I'm missing here?