Saubermfg,
I have never set anything like this up before so I don't have much faith in this working, but this is what I would try to do without changing your original setup too much.
Create a Call Routing Announcement, Set the Day and Night Greetings to Nothing, Set the Digit Translation Timeout to the Main MBX
Create a new Hunt Group.
Create an Extension List with extensions D, E, F, and G
Put the Extension List in the new Hunt Group
Set the Recall to the Call Routing Announcement
Set the Recall timer to how ever long you want the call to go unanswered with users available before sending it to the new Call Routing Announcement for the Main Mailbox to answer.
Set the original Hunt Group Recall to the new Hunt Group extension
Set the Recall timer to how ever long you want the call to go unanswered with users available before sending it to the new Hunt Group to get answered.
If this doesn't work than I would try to figure out what the customer is really trying to accomplish with this scenario and find a solution that would accomplish the end goal.
Thanks,
TE