Not the worst case I've seen by a long way LeeH. Ivermectin carries an egg withdrawal period of up to 28 days, depending on your vet, because it is unlicensed for poultry so I'd be very reluctant to use it -ever.
Ivomec has no meat withdrawal period for cattle, and as you correctly say, the withdrawal period for poultry is at the vet's discretion, merely because there is no licence information to the contrary.
However, there were tests done in the States that indicated no residue found in eggs after Ivomec was applied externally at the recommended dose. (You need to extrapolate down from the cattle dose, and it equates to between 1 and 5 drops/bird.)
I'm afraid I haven't got the link but if you google "Ivomec withdrawal poultry" you will find it.
I use it on my own poultry because it not only treats scaly leg mite, but red mite as well. And I carry on eating the eggs.