[Exim] matching sender_helo_name in a named list

I'm using exim 4.32. At the top of my configuration, I have the
following:

domainlist local_domains = mysql;select distinct domain from users \
where domain='$domain';

I then use the named list "local_domains" in various tests in the RCPT
ACL.

One of the tests I want to do is to see if $sender_helo_name matches
any of the domain in "local_domains", and if so, to reject the RCPTs,
because the sender should not be using one of my local domain names in
the HELO. This check happens after the check for authentication, so my
users won't have a problem, even if they use silly mail clients like
Thunderbird (which insists on using the domain name in one's email
address in the HELO command).

So, how can I do this test? I've looked at the documentation, and I
can't seem to figure out a way of doing this (I could write my local
domains into a file, and do a lookup, but I want to avoid that,
because then the mysql DB and the file have to be kept in sync).

--
Anand Buddhdev
Celtel International