[Exim] custom reason for a forced verify failure

I'm using exim as my mail gateway for our network, we have several
internal machines where the mail ends up.
But theese internal machines are running "First Class", and they don't
know how to generate a sane bounce message.

So I've set up exim as per 0822 in the FAQ (with file lookups) to reject
certain users, people who have quit here and are still on several
"announcement lists" (a few of them with non valid return paths).

I want to do this direct at smtp level to stop the message from entering
my systems at all.

The problem is that some of the more recently deleted accounts still
get "real" mail sent here, and the senders of those mails get really
confused from the bounces, since the rejections look like this:

550 Cannot route to <foo.bar@???>

I would like to change that to:

550 User doesn't exist <foo.bar@???>

So I really would like to be able to have a custom reason for the
failure, as with the prohibition_message option, but this doesn't
seem to be possible with a receiver verification.
I can't find any prohibition_reason for receiver_verification and my
debug sessions shows that prohibition_message is never checked when
the verify fails.
So is there any way of doing what i want?

/Andreas Pettersson