Does anyone know if you can mass update item fulfillments? I have a new custom field I want to populate retroactively with data...

Yes it’s listed under List->Mass Updates->General Updates->Transactions

Thanks! I didn't know about that one... well unfortunately there's not enough control there that I need for conditional selects and conditional values. I was hoping for more of a CSV-style one where I could externally set the value and just update it from a static list. I'll keep this in my back pocket though...

You can create a mass update script to manage any conditions you need and use that to run the mass update.