[Exim] Override server callout verification is done on?

I have possibly a weird wishlist item - the ability to specify a server that
a verification callout is sent to.

What I mean by this is rather than have Exim do a DNS lookup, get the MX
record, then connect and try to do a sender/recip verification callout, I'd
like to just specify a single host/IP that would be used for the check.

The reason for this is to have Exim as a front end to a backend mail server,
but Exim won't have any knowledge of the userbase, so I want to do a callout
to the backend system to verify if the user account exists or not. I know
it's not the best of set ups, but maybe this may even have other uses better
than this scenario?

Just wondering if it's even worth requesting, or if there are too many
issues that could arise from having such an ability?

Eli.