ALL nodes running exactly the same software...
That's a reasonable requirement and doable and I understand why he wants to get away from having hardcoded and unique values programmed into each node, especially if this will be a product where maybe you don't know in advance how many will be plugged in and holding inventory of nodes with hardcoded addresses would be difficult.
I have a similar requirement at present (not with I2C) but the only way I know to do it is to bring the nodes online one at a time or "by controlled sequential startup of the slaves" as you say. As this is a one-off thing when you first build a system I think that's a reasonable approach.
Rob