Today we managed to identify the cause of our original issue.
We had printer 1 set with a static IP, and this same IP set as a reservation on DHCP for printer 2.
If Printer1 was switched on first, and then Printer2 switched on, a cycle of DHCP requests and NAK's started, we found that our network was working flat out to service them, and our DHCP server was using 10% of a 100Mb link, but still able to service requests.
Unsurprisingly, when the backup ran, the DHCP service hung while shutting down leading to an outage.
The feature request would still be useful for those one in a million events like the server crashing mid-backup etc, and of course for human error.