It seems to me that the second time that Icheb's parents sent him up to be Borg bait, that the Borg would become wise to the plot, as it takes awhile for the Virus to take effect after he gets assimilated again. The Collective would get his memories of what happened on the cube the first time he was assimilated and they'd put two and two together. I don't imagine the Brunali trying that trick again with someone else would work again.