[Exim] Negating a lookup in host_lookup

Hi folks,

Is it possible to negate the sense of a lookup?

I'm providing a pop- before-smtp relaying service to my POP3 account
holders. Unlike incoming SMTP sessions from remote mail servers, where
a few seconds for a slow DNS lookup is nothing, SMTP relay sessions for
these people need to be as fast as possible.

I'd like to suppress host_lookup for these users. I was hoping I might
get away with something like this:

MYSQL_RELAY = select ip from popb4smtp where ip="${sender_host_address}"
host_accept_relay = 127.0.0.1/32 : 10.0.0.1/24 : mysql;MYSQL_RELAY

[...]

host_lookup = !mysql;MYSQL_RELAY

The lookup documentation doesn't say you can do this, so there's no
reason for me to think it should work. I'm just hoping someone can tell
me I'm being stupid and have overlooked an easy way to accomplish this.
:-)

Thanks,
Sheldon.