[exim] How to defer (ie deliberately delay) delivery

Greetings,

We are using Exim as a MTA to handle mails from applications (eg cron
jobs), with all mail directed via a smarthost (so exim doesn't connect
directly to the internet).

A user has requested a feature whereby he can *delay* the delivery of
a mail until a fixed time (eg 22.00h). The idea is that the mail is
generated asynchronously during the day and submitted immediately to
exim, but with a header like "Deliver-At: 22.00". The mail should then
sit in the queue until the first queue-run after 22.00h. Then we
should detect it and send it to the smart-host for final delivery.

I have checked the documentation and the mailing-list archives, but
can't see anything that looks like it might do this. Any
suggestions/pointers?

Best regards,
Owen Boyle