Milter-greylist

Has anyone done anything with doing greylisting only during certain 
times of the day?

We really like the greylisting idea, but have come across too many 
poorly configured or implemented servers that wait much more than the 
time suggested.  Some of them are waiting up to 6 hours apparently.  
During the day, this just isn't going to work for us -- we can't have 
mail for employees being delayed that long (even just once).

However, a significant portion of the spam that we get seems to come 
in the off-hours.  Therefore, I think that, for us, a good option 
might be to greylist only from, say, 7pm until 6am -- and then do no 
greylisting during normal business hours.

Is anyone doing anything like this?  I'd really like to see something 
like this in the greylist configuration -- but, as a temporary 
workaround method, it should work just to do cron jobs to start and 
stop the milter, correct?  With the milter stopped, sendmail will 
just process the mail normally, correct?

 - John...