I think normally when you choose to, or are asked to, "______ your head off," it's an action that is naturally head-related, like talking or laughing for example. I've never heard of "work[ing] your head off." Let's try to think of a suitable alternative, shall we? Great, go...
- Work your tail off?
- Work your body off?
- Work your hair flat?
- Work your crewneck blue?
- Work your deer into a proper foster home?