If you are caught out, the hotel will be well within their rights to charge you the full rack rate on your arrival, which would be far more expensive than if you had chosen a regular prepaid rate.

Family and friends rate does generally require a copy of the staff member's staff card to be sent to the hotel prior to the stay. This is because the staff member is essentially "on the hook" if their family/friends end up being bad guests.

This is also why I doubt any Accor staff who may frequent the forum would willingly "sponsor" a stranger for a family and friends stay.

I also doubt that these stays would receive status recognition or accrue points/nights, without knowing for sure.