[Exim] QUEUE all messages

Hello,

I am planning for disasters and this question is concerning the state when
exim is running but mysql is down. I am curious how I can setup exim to
queue all messages for any domain, and sort out what bounces, and what gets
thru once mysql is restored. The problem for me is as follows:

a) assuming mysql is not available, how can I queue instead of lookup when
mysql is not around for the local_domains. I tried:

domainlist local_domains = @:${lookup mysql {SELECT domainName FROM domain
WHERE domainName='$domain'} {$value}{$domain}}

But the problem here is that I want to disallow relaying normally (i.e, the
lookup failed to find a corresponding virtual domain), but when mysql is
unavailable, allow anything to queue.

b) all my routers require a DB lookup for domains = ; assuming mysql is not
around, I try all routers (and their lookups fail) eventually falling thru
and rejecting with 451.

I looked at queue_only and hold_domains=$domain, but because of (b) above, I
don't get to queue until a suitable router has been found. Is there a way to
simply accept *everything* without running any checks on routers /
local_domains, etc, and then, once mysql has been restored, run the queued
messages thru the 'rules'?

Thanks,
--Eric