[Exim] local_delivery transport_filter

I've been running SpamAssassin as a transport_filter for local_delivery,
just to tag the e-mails. This allows each user to have their own
customized settings, even when there are multiple recipients.

local_delivery:
driver = appendfile
directory = $home/Maildir
maildir_format
maildir_tag = ,S=$message_size
delivery_date_add
envelope_to_add
return_path_add
transport_filter = /usr/bin/spamc

This works great, for tagging. But I'm thinking about delivering
messages tagged as spam into "directory = $home/Maildir/.spam". There
is no way to pull the headers that SpamAssassin adds in the
transport_filter to use in a varible expantion in the transport that
runs it, is there? That is, if X-Spam-Flag exists, deliver into
~/Maildir/.spam, else ~/Maildir.

Is there another way to have spamc run as the local user that the mail
is going to be delivered to before it actually reaches the delivery
transport?

--
Chris