[Exim] $value in hosts= acl

I've been playing around w/ various DNSBL services for a while, and
finally decided that I'd rather implement it as a cdb file lookup than a
local DNS zone. spfilter lets me create files w/ contents like this:

12.0.195.78:DSBL Insecure host http://dsbl.org/listing?12.0.195.78

I want to flag the message as tripping this, and provide the TXT record
"Insecure host..." as part of the header. This was a breeze when I was
doing DNS lookups (which had special variables), but doesn't seem to be
working for file lookup. Here's my ACL:

  warn message = X-RBL-hit-at: $value
       log_message = RBL-hit $value
       hosts = net-cdb;GRULESD/cdb.rblhosts.cdb

This works in that the X-RBL-hit-at: header gets added to the mail, but
it's always empty:

Subject: test flatfile lookup
Date: Thu, 10 Apr 2003 13:41:49 -0500
X-RBL-hit-at:

test

What'm I missing? If $value is the wrong variable, is there a right one?

Thanks
--John