Yes, we've gone horribly off-topic.. but here's what I got...

Senior management staff hired before 1986 is allowed to purchase staff discounts on SQ in premium classes.

Captains on duty (in and out of uniform) are allowed to travel in F if there are empty seats available.

Crew cannot 'buddy-buddy' and ask to be placed in F after check-in. Most likely, even if it is an upgrade, there has to be a logical reason for the station manager to ok it otherwise he will have to answer to senior management at the airport as to why he ok'd that. Cabin crew cannot also place their collegues in premium classes after the door closes.. unless the IFS says it's ok. But even so, someone else in the crew compliment may complain to their management and that IFS will be in trouble. So in other words.. it's really hard for staff to get upgraded.. unless they are staff on company business, then if it's an op-up due to oversell, then it may happen.