Using $set on custom field

I'm creating a custom field. I really need the data to be set dynamically from an afterStateUpdated call buried in a repeater. Is there any reason that wouldn't work? Is there something special I need to do in the component? Everything seems to be working, I can use the component in ordinary circumstances and I'm sure the ../../ scope is correct but the data never gets updated. Everything seems to be working fine except the $set.